I want a simple C method to be able to run hex bytecode on a Linux 64 bit machine. Here's the C program that I have:
char code[] = "\x48\x31\xc0";
#include <stdio.h>
int main(int argc, char **argv)
{
int (*func) ();
func = (int (*)()) code;
(int)(*func)();
printf("%s\n","DONE");
}
The code that I am trying to run ("\x48\x31\xc0") I obtained by writting this simple assembly program (it's not supposed to really do anything)
.text
.globl _start
_start:
xorq %rax, %rax
and then compiling and objdump-ing it to obtain the bytecode.
However, when I run my C program I get a segmentation fault. Any ideas?
Machine code has to be in an executable page. Your char code[] is in the read+write data section, without exec permission, so the code cannot be executed from there.
Here is a simple example of allocating an executable page with mmap:
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
int main ()
{
char code[] = {
0x8D, 0x04, 0x37, // lea eax,[rdi+rsi]
0xC3 // ret
};
int (*sum) (int, int) = NULL;
// allocate executable buffer
sum = mmap (0, sizeof(code), PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
// copy code to buffer
memcpy (sum, code, sizeof(code));
// doesn't actually flush cache on x86, but ensure memcpy isn't
// optimized away as a dead store.
__builtin___clear_cache (sum, sum + sizeof(sum)); // GNU C
// run code
int a = 2;
int b = 3;
int c = sum (a, b);
printf ("%d + %d = %d\n", a, b, c);
}
See another answer on this question for details about __builtin___clear_cache.
Until recent Linux kernel versions (sometime before 5.4), you could simply compile with gcc -z execstack - that would make all pages executable, including read-only data (.rodata), and read-write data (.data) where char code[] = "..." goes.
Now -z execstack only applies to the actual stack, so it currently works only for non-const local arrays. i.e. move char code[] = ... into main.
See Linux default behavior against `.data` section for the kernel change, and Unexpected exec permission from mmap when assembly files included in the project for the old behaviour: enabling Linux's READ_IMPLIES_EXEC process for that program. (In Linux 5.4, that Q&A shows you'd only get READ_IMPLIES_EXEC for a missing PT_GNU_STACK, like a really old binary; modern GCC -z execstack would set PT_GNU_STACK = RWX metadata in the executable, which Linux 5.4 would handle as making only the stack itself executable. At some point before that, PT_GNU_STACK = RWX did result in READ_IMPLIES_EXEC.)
The other option is to make system calls at runtime to copy into an executable page, or change permissions on the page it's in. That's still more complicated than using a local array to get GCC to copy code into executable stack memory.
(I don't know if there's an easy way to enable READ_IMPLIES_EXEC under modern kernels. Having no GNU-stack attribute at all in an ELF binary does that for 32-bit code, but not 64-bit.)
Yet another option is __attribute__((section(".text"))) const char code[] = ...;
Working example: https://godbolt.org/z/draGeh.
If you need the array to be writeable, e.g. for shellcode that inserts some zeros into strings, you could maybe link with ld -N. But probably best to use -z execstack and a local array.
Two problems in the question:
exec permission on the page, because you used an array that will go in the noexec read+write .data section.
your machine code doesn't end with a ret instruction so even if it did run, execution would fall into whatever was next in memory instead of returning.
And BTW, the REX prefix is totally redundant. "\x31\xc0" xor eax,eax has exactly the same effect as xor rax,rax.
You need the page containing the machine code to have execute permission. x86-64 page tables have a separate bit for execute separate from read permission, unlike legacy 386 page tables.
The easiest way to get static arrays to be in read+exec memory was to compile with gcc -z execstack. (Used to make the stack and other sections executable, now only the stack).
Until recently (2018 or 2019), the standard toolchain (binutils ld) would put section .rodata into the same ELF segment as .text, so they'd both have read+exec permission. Thus using const char code[] = "..."; was sufficient for executing manually-specified bytes as data, without execstack.
But on my Arch Linux system with GNU ld (GNU Binutils) 2.31.1, that's no longer the case. readelf -a shows that the .rodata section went into an ELF segment with .eh_frame_hdr and .eh_frame, and it only has Read permission. .text goes in a segment with Read + Exec, and .data goes in a segment with Read + Write (along with the .got and .got.plt). (What's the difference of section and segment in ELF file format)
I assume this change is to make ROP and Spectre attacks harder by not having read-only data in executable pages where sequences of useful bytes could be used as "gadgets" that end with the bytes for a ret or jmp reg instruction.
// TODO: use char code[] = {...} inside main, with -z execstack, for current Linux
// Broken on recent Linux, used to work without execstack.
#include <stdio.h>
// can be non-const if you use gcc -z execstack. static is also optional
static const char code[] = {
0x8D, 0x04, 0x37, // lea eax,[rdi+rsi] // retval = a+b;
0xC3 // ret
};
static const char ret0_code[] = "\x31\xc0\xc3"; // xor eax,eax ; ret
// the compiler will append a 0 byte to terminate the C string,
// but that's fine. It's after the ret.
int main () {
// void* cast is easier to type than a cast to function pointer,
// and in C can be assigned to any other pointer type. (not C++)
int (*sum) (int, int) = (void*)code;
int (*ret0)(void) = (void*)ret0_code;
// run code
int c = sum (2, 3);
return ret0();
}
On older Linux systems: gcc -O3 shellcode.c && ./a.out (Works because of const on global/static arrays)
On Linux before 5.5 (or so) gcc -O3 -z execstack shellcode.c && ./a.out (works because of -zexecstack regardless of where your machine code is stored). Fun fact: gcc allows -zexecstack with no space, but clang only accepts clang -z execstack.
These also work on Windows, where read-only data goes in .rdata instead of .rodata.
The compiler-generated main looks like this (from objdump -drwC -Mintel). You can run it inside gdb and set breakpoints on code and ret0_code
(I actually used gcc -no-pie -O3 -zexecstack shellcode.c hence the addresses near 401000
0000000000401020 <main>:
401020: 48 83 ec 08 sub rsp,0x8 # stack aligned by 16 before a call
401024: be 03 00 00 00 mov esi,0x3
401029: bf 02 00 00 00 mov edi,0x2 # 2 args
40102e: e8 d5 0f 00 00 call 402008 <code> # note the target address in the next page
401033: 48 83 c4 08 add rsp,0x8
401037: e9 c8 0f 00 00 jmp 402004 <ret0_code> # optimized tailcall
Or use system calls to modify page permissions
Instead of compiling with gcc -zexecstack, you can instead use mmap(PROT_EXEC) to allocate new executable pages, or mprotect(PROT_EXEC) to change existing pages to executable. (Including pages holding static data.) You also typically want at least PROT_READ and sometimes PROT_WRITE, of course.
Using mprotect on a static array means you're still executing the code from a known location, maybe making it easier to set a breakpoint on it.
On Windows you can use VirtualAlloc or VirtualProtect.
Telling the compiler that data is executed as code
Normally compilers like GCC assume that data and code are separate. This is like type-based strict aliasing, but even using char* doesn't make it well-defined to store into a buffer and then call that buffer as a function pointer.
In GNU C, you also need to use __builtin___clear_cache(buf, buf + len) after writing machine code bytes to a buffer, because the optimizer doesn't treat dereferencing a function pointer as reading bytes from that address. Dead-store elimination can remove the stores of machine code bytes into a buffer, if the compiler proves that the store isn't read as data by anything. https://codegolf.stackexchange.com/questions/160100/the-repetitive-byte-counter/160236#160236 and https://godbolt.org/g/pGXn3B has an example where gcc really does do this optimization, because gcc "knows about" malloc.
(And on non-x86 architectures where I-cache isn't coherent with D-cache, it actually will do any necessary cache syncing. On x86 it's purely a compile-time optimization blocker and doesn't expand to any instructions itself.)
Re: the weird name with three underscores: It's the usual __builtin_name pattern, but name is __clear_cache.
My edit on #AntoineMathys's answer added this.
In practice GCC/clang don't "know about" mmap(MAP_ANONYMOUS) the way they know about malloc. So in practice the optimizer will assume that the memcpy into the buffer might be read as data by the non-inline function call through the function pointer, even without __builtin___clear_cache(). (Unless you declared the function type as __attribute__((const)).)
On x86, where I-cache is coherent with data caches, having the stores happen in asm before the call is sufficient for correctness. On other ISAs, __builtin___clear_cache() will actually emit special instructions as well as ensuring the right compile-time ordering.
It's good practice to include it when copying code into a buffer because it doesn't cost performance, and stops hypothetical future compilers from breaking your code. (e.g. if they do understand that mmap(MAP_ANONYMOUS) gives newly-allocated anonymous memory that nothing else has a pointer to, just like malloc.)
With current GCC, I was able to provoke GCC into really doing an optimization we don't want by using __attribute__((const)) to tell the optimizer sum() is a pure function (that only reads its args, not global memory). GCC then knows sum() can't read the result of the memcpy as data.
With another memcpy into the same buffer after the call, GCC does dead-store elimination into just the 2nd store after the call. This results in no store before the first call so it executes the 00 00 add [rax], al bytes, segfaulting.
// demo of a problem on x86 when not using __builtin___clear_cache
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
int main ()
{
char code[] = {
0x8D, 0x04, 0x37, // lea eax,[rdi+rsi]
0xC3 // ret
};
__attribute__((const)) int (*sum) (int, int) = NULL;
// copy code to executable buffer
sum = mmap (0,sizeof(code),PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANON,-1,0);
memcpy (sum, code, sizeof(code));
//__builtin___clear_cache(sum, sum + sizeof(code));
int c = sum (2, 3);
//printf ("%d + %d = %d\n", a, b, c);
memcpy(sum, (char[]){0x31, 0xc0, 0xc3, 0}, 4); // xor-zero eax, ret, padding for a dword store
//__builtin___clear_cache(sum, sum + 4);
return sum(2,3);
}
Compiled on the Godbolt compiler explorer with GCC9.2 -O3
main:
push rbx
xor r9d, r9d
mov r8d, -1
mov ecx, 34
mov edx, 7
mov esi, 4
xor edi, edi
sub rsp, 16
call mmap
mov esi, 3
mov edi, 2
mov rbx, rax
call rax # call before store
mov DWORD PTR [rbx], 12828721 # 0xC3C031 = xor-zero eax, ret
add rsp, 16
pop rbx
ret # no 2nd call, CSEd away because const and same args
Passing different args would have gotten another call reg, but even with __builtin___clear_cache the two sum(2,3) calls can CSE. __attribute__((const)) doesn't respect changes to the machine code of a function. Don't do it. It's safe if you're going to JIT the function once and then call many times, though.
Uncommenting the first __clear_cache results in
mov DWORD PTR [rax], -1019804531 # lea; ret
call rax
mov DWORD PTR [rbx], 12828721 # xor-zero; ret
... still CSE and use the RAX return value
The first store is there because of __clear_cache and the sum(2,3) call. (Removing the first sum(2,3) call does let dead-store elimination happen across the __clear_cache.)
The second store is there because the side-effect on the buffer returned by mmap is assumed to be important, and that's the final value main leaves.
Godbolt's ./a.out option to run the program still seems to always fail (exit status of 255); maybe it sandboxes JITing? It works on my desktop with __clear_cache and crashes without.
mprotect on a page holding existing C variables.
You can also give a single existing page read+write+exec permission. This is an alternative to compiling with -z execstack
You don't need __clear_cache on a page holding read-only C variables because there's no store to optimize away. You would still need it for initializing a local buffer (on the stack). Otherwise GCC will optimize away the initializer for this private buffer that a non-inline function call definitely doesn't have a pointer to. (Escape analysis). It doesn't consider the possibility that the buffer might hold the machine code for the function unless you tell it that via __builtin___clear_cache.
#include <stdio.h>
#include <sys/mman.h>
#include <stdint.h>
// can be non-const if you want, we're using mprotect
static const char code[] = {
0x8D, 0x04, 0x37, // lea eax,[rdi+rsi] // retval = a+b;
0xC3 // ret
};
static const char ret0_code[] = "\x31\xc0\xc3";
int main () {
// void* cast is easier to type than a cast to function pointer,
// and in C can be assigned to any other pointer type. (not C++)
int (*sum) (int, int) = (void*)code;
int (*ret0)(void) = (void*)ret0_code;
// hard-coding x86's 4k page size for simplicity.
// also assume that `code` doesn't span a page boundary and that ret0_code is in the same page.
uintptr_t page = (uintptr_t)code & -4095ULL; // round down
mprotect((void*)page, 4096, PROT_READ|PROT_EXEC|PROT_WRITE); // +write in case the page holds any writeable C vars that would crash later code.
// run code
int c = sum (2, 3);
return ret0();
}
I used PROT_READ|PROT_EXEC|PROT_WRITE in this example so it works regardless of where your variable is. If it was a local on the stack and you left out PROT_WRITE, call would fail after making the stack read only when it tried to push a return address.
Also, PROT_WRITE lets you test shellcode that self-modifies, e.g. to edit zeros into its own machine code, or other bytes it was avoiding.
$ gcc -O3 shellcode.c # without -z execstack
$ ./a.out
$ echo $?
0
$ strace ./a.out
...
mprotect(0x55605aa3f000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC) = 0
exit_group(0) = ?
+++ exited with 0 +++
If I comment out the mprotect, it does segfault with recent versions of GNU Binutils ld which no longer put read-only constant data into the same ELF segment as the .text section.
If I did something like ret0_code[2] = 0xc3;, I would need __builtin___clear_cache(ret0_code+2, ret0_code+2) after that to make sure the store wasn't optimized away, but if I don't modify the static arrays then it's not needed after mprotect. It is needed after mmap+memcpy or manual stores, because we want to execute bytes that have been written in C (with memcpy).
You need to include the assembly in-line via a special compiler directive so that it'll properly end up in a code segment. See this guide, for example: http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html
Your machine code may be all right, but your CPU objects.
Modern CPUs manage memory in segments. In normal operation, the operating system loads a new program into a program-text segment and sets up a stack in a data segment. The operating system tells the CPU never to run code in a data segment. Your code is in code[], in a data segment. Thus the segfault.
This will take some effort.
Your code variable is stored in the .data section of your executable:
$ readelf -p .data exploit
String dump of section '.data':
[ 10] H1À
H1À is the value of your variable.
The .data section is not executable:
$ readelf -S exploit
There are 30 section headers, starting at offset 0x1150:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[...]
[24] .data PROGBITS 0000000000601010 00001010
0000000000000014 0000000000000000 WA 0 0 8
All 64-bit processors I'm familiar with support non-executable pages natively in the pagetables. Most newer 32-bit processors (the ones that support PAE) provide enough extra space in their pagetables for the operating system to emulate hardware non-executable pages. You'll need to run either an ancient OS or an ancient processor to get a .data section marked executable.
Because these are just flags in the executable, you ought to be able to set the X flag through some other mechanism, but I don't know how to do so. And your OS might not even let you have pages that are both writable and executable.
You may need to set the page executable before you may call it.
On MS-Windows, see the VirtualProtect -function.
URL: http://msdn.microsoft.com/en-us/library/windows/desktop/aa366898%28v=vs.85%29.aspx
Sorry, I couldn't follow above examples which are complicated.
So, I created an elegant solution for executing hex code from C.
Basically, you could use asm and .word keywords to place your instructions in hex format.
See below example:
asm volatile(".rept 1024\n"
CNOP
".endr\n");
where CNOP is defined as below:
#define ".word 0x00010001 \n"
Basically, c.nop instruction was not supported by my current assembler. So, I defined CNOP as the hex equivalent of c.nop with proper syntax and used inside asm, with which I was aware of.
.rept <NUM> .endr will basically, repeat the instruction NUM times.
This solution is working and verified.
this is a simple linux kernel module code to reverse a string which should Oops after insmod,but it works well,why?
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
static char *words = "words";
static int __init words_init(void)
{
printk(KERN_INFO "debug info\n");
int len = strlen(words);
int k;
for ( k = 0; k < len/2; k++ )
{
printk("the value of k %d\n",k);
char a = words[k];
words[k]= words[len-1-k];
words[len-1-k]=a;
}
printk(KERN_INFO "words is %s\n", words);
return 0;
}
static void __exit words_exit(void)
{
printk(KERN_INFO "words exit.\n");
}
module_init(words_init);
module_exit(words_exit);
module_param(words, charp, S_IRUGO);
MODULE_LICENSE("GPL");
static != const.
static in your example means "only visible in the current file". See What does "static" mean?
const just means "the code which can see this declaration isn't supposed to change the variable". But in certain cases, you can still create a non-const pointer to the same address and modify the contents.
Space removal from a String - In Place C style with Pointers suggests that writing to the string value shouldn't be possible.
Since static is the only difference between your code and that of the question, I looked further and found this: Global initialized variables declared as "const" go to text segment, while those declared "Static" go to data segment. Why?
Try static const char *word, that should do the trick.
Haha!,I get the answer from the linux kernel source code by myself;When you use the insmod,it will call the init_moudle,load_module,strndup_usr then memdup_usr function;the memdup_usr function will use kmalloc_track_caller to alloc memery from slab and then use the copy_from_usr to copy the module paragram into kernel;this mean the linux kernel module paragram store in heap,not in the constant storage area!! So we can change it's content!
I would like to use ld's --build-id option in order to add build information to my binary. However, I'm not sure how to make this information available inside the program. Assume I want to write a program that writes a backtrace every time an exception occurs, and a script that parses this information. The script reads the symbol table of the program and searches for the addresses printed in the backtrace (I'm forced to use such a script because the program is statically linked and backtrace_symbols is not working). In order for the script to work correctly I need to match build version of the program with the build version of the program which created the backtrace. How can I print the build version of the program (located in the .note.gnu.build-id elf section) from the program itself?
How can I print the build version of the program (located in the .note.gnu.build-id elf section) from the program itself?
You need to read the ElfW(Ehdr) (at the beginning of the file) to find program headers in your binary (.e_phoff and .e_phnum will tell you where program headers are, and how many of them to read).
You then read program headers, until you find PT_NOTE segment of your program. That segment will tell you offset to the beginning of all the notes in your binary.
You then need to read the ElfW(Nhdr) and skip the rest of the note (total size of the note is sizeof(Nhdr) + .n_namesz + .n_descsz, properly aligned), until you find a note with .n_type == NT_GNU_BUILD_ID.
Once you find NT_GNU_BUILD_ID note, skip past its .n_namesz, and read the .n_descsz bytes to read the actual build-id.
You can verify that you are reading the right data by comparing what you read with the output of readelf -n a.out.
P.S.
If you are going to go through the trouble to decode build-id as above, and if your executable is not stripped, it may be better for you to just decode and print symbol names instead (i.e. to replicate what backtrace_symbols does) -- it's actually easier to do than decoding ELF notes, because the symbol table contains fixed-sized entries.
Basically, this is the code I've written based on answer given to my question. In order to compile the code I had to make some changes and I hope it will work for as many types of platforms as possible. However, it was tested only on one build machine. One of the assumptions I used was that the program was built on the machine which runs it so no point in checking endianness compatibility between the program and the machine.
user#:~/$ uname -s -r -m -o
Linux 3.2.0-45-generic x86_64 GNU/Linux
user#:~/$ g++ test.cpp -o test
user#:~/$ readelf -n test | grep Build
Build ID: dc5c4682e0282e2bd8bc2d3b61cfe35826aa34fc
user#:~/$ ./test
Build ID: dc5c4682e0282e2bd8bc2d3b61cfe35826aa34fc
#include <elf.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#if __x86_64__
# define ElfW(type) Elf64_##type
#else
# define ElfW(type) Elf32_##type
#endif
/*
detecting build id of a program from its note section
http://stackoverflow.com/questions/17637745/can-a-program-read-its-own-elf-section
http://www.scs.stanford.edu/histar/src/pkg/uclibc/utils/readelf.c
http://www.sco.com/developers/gabi/2000-07-17/ch5.pheader.html#note_section
*/
int main (int argc, char* argv[])
{
char *thefilename = argv[0];
FILE *thefile;
struct stat statbuf;
ElfW(Ehdr) *ehdr = 0;
ElfW(Phdr) *phdr = 0;
ElfW(Nhdr) *nhdr = 0;
if (!(thefile = fopen(thefilename, "r"))) {
perror(thefilename);
exit(EXIT_FAILURE);
}
if (fstat(fileno(thefile), &statbuf) < 0) {
perror(thefilename);
exit(EXIT_FAILURE);
}
ehdr = (ElfW(Ehdr) *)mmap(0, statbuf.st_size,
PROT_READ|PROT_WRITE, MAP_PRIVATE, fileno(thefile), 0);
phdr = (ElfW(Phdr) *)(ehdr->e_phoff + (size_t)ehdr);
while (phdr->p_type != PT_NOTE)
{
++phdr;
}
nhdr = (ElfW(Nhdr) *)(phdr->p_offset + (size_t)ehdr);
while (nhdr->n_type != NT_GNU_BUILD_ID)
{
nhdr = (ElfW(Nhdr) *)((size_t)nhdr + sizeof(ElfW(Nhdr)) + nhdr->n_namesz + nhdr->n_descsz);
}
unsigned char * build_id = (unsigned char *)malloc(nhdr->n_descsz);
memcpy(build_id, (void *)((size_t)nhdr + sizeof(ElfW(Nhdr)) + nhdr->n_namesz), nhdr->n_descsz);
printf(" Build ID: ");
for (int i = 0 ; i < nhdr->n_descsz ; ++i)
{
printf("%02x",build_id[i]);
}
free(build_id);
printf("\n");
return 0;
}
Yes, a program can read its own .note.gnu.build-id. The important piece is the dl_iterate_phdr function.
I've used this technique in Mesa (the OpenGL/Vulkan implementation) to read its own build-id for use with the on-disk shader cache.
I've extracted those bits into a separate project[1] for easy use by others.
[1] https://github.com/mattst88/build-id
In Linux. Per the dlsym(3) Linux man page,
*Since the value of the symbol could actually be NULL
(so that a NULL return from dlsym() need not indicate an error),*
Why is this, when can a symbol (for a function, specifically) be actually NULL? I am reviewing code and found a piece using dlerror to clean first, dlsym next, and dlerror to check for errors. But it does not check the resulting function from being null before calling it:
dlerror();
a_func_name = ...dlsym(...);
if (dlerror()) goto end;
a_func_name(...); // Never checked if a_func_name == NULL;
I am just a reviewer so don't have the option to just add the check. And perhaps the author knows NULL can never be returned. My job is to challenge that but don't know what could make this return a valid NULL so I can then check if such a condition could be met in this code's context. Have not found the right thing to read with Google, a pointer to good documentation would be enough unless you want to explain explicitly which would be great.
I know of one particular case where the symbol value returned by dlsym() can be NULL, which is when using GNU indirection functions (IFUNCs). However, there are presumably other cases, since the text in the dlsym(3) manual page pre-dates the invention of IFUNCs.
Here's an example using IFUNCs. First, a file that will be used to create a shared library:
$ cat foo.c
/* foo.c */
#include <stdio.h>
/* This is a 'GNU indirect function' (IFUNC) that will be called by
dlsym() to resolve the symbol "foo" to an address. Typically, such
a function would return the address of an actual function, but it
can also just return NULL. For some background on IFUNCs, see
https://willnewton.name/uncategorized/using-gnu-indirect-functions/ */
asm (".type foo, #gnu_indirect_function");
void *
foo(void)
{
fprintf(stderr, "foo called\n");
return NULL;
}
Now the main program, which will look up the symbol foo in the shared library:
$ cat main.c
/* main.c */
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
void *handle;
void (*funcp)(void);
handle = dlopen("./foo.so", RTLD_LAZY);
if (handle == NULL) {
fprintf(stderr, "dlopen: %s\n", dlerror());
exit(EXIT_FAILURE);
}
dlerror(); /* Clear any outstanding error */
funcp = dlsym(handle, "foo");
printf("Results after dlsym(): funcp = %p; dlerror = %s\n",
(void *) funcp, dlerror());
exit(EXIT_SUCCESS);
}
Now build and run to see a case where dlsym() returns NULL, while dlerror() also returns NULL:
$ cc -Wall -fPIC -shared -o libfoo.so foo.c
$ cc -Wall -o main main.c libfoo.so -ldl
$ LD_LIBRARY_PATH=. ./main
foo called
Results after dlsym(): funcp = (nil); dlerror = (null)
Well, if it's returned with no errors, then pointer is valid and NULL is about as illegal as any random pointer from the shared object. Like the wrong function, data or whatever.
It can't be if the library/PIE is a product of normal C compilation, as C won't ever put a global object at the NULL address, but you can get a symbol to resolve to NULL using special linker tricks:
null.c:
#include <stdio.h>
extern char null_addressed_char;
int main(void)
{
printf("&null_addressed_char=%p\n", &null_addressed_char);
}
Compile, link, and run:
$ clang null.c -Xlinker --defsym -Xlinker null_addressed_char=0 && ./a.out
&null_addressed_char=(nil)
If you don't allow any such weirdness, you can treat NULL returns from dlsym as errors.
dlerror() returns the last error, not the status of the last call. So if nothing else the code you show may potentially get a valid result from dlsym() and fool itself into thinking there was an error (because there was still one in the queue). The purpose behind dlerror is to provide human-readable error messages. If you aren't printing the result, you are using it wrong.