I have a question about assert() in Linux: can I use it in the kernel?
If no, what techniques do you usually use if, for example I don't want to enter NULL pointer?
The corresponding kernel macros are BUG_ON and WARN_ON. The former is for when you want to make the kernel panic and bring the system down (i.e., unrecoverable error). The latter is for when you want to log something to the kernel log (viewable via dmesg).
As #Michael says, in the kernel, you need to validate anything that comes from userspace and just handle it, whatever it is. BUG_ON and WARN_ON are to catch bugs in your own code or problems with the hardware.
One option would be to use the macro BUG_ON(). It will printk a message, and then panic() (i.e. crash) the kernel.
http://kernelnewbies.org/KernelHacking-HOWTO/Debugging_Kernel
Of course, this should only be used as an error handling strategy of last resort (just like assert)...
No. Unless you're working on the kernel core and rather on a module, you should do your best to never crash (technically, abort()) the kernel. If you don't want to use a NULL pointer, just don't do it. Check it before using it, and produce an error log if it is.
The closest thing you might want to do if you're actually handling a fatal case is the panic() function or the BUG_ON and WARN_ON macros, which will abort execution and produce diagnostic messages, a stack trace and a list of modules.
Well, dereferencing null pointer will produce an oops, which you can use to find the offending code. Now, if you want to assert() a given condition, you can use
BUG_ON(condition)
A less lethal mechanism is WARN_ON, which will produce a backtrace without crashing the kernel.
I use this macro, it uses BUG() but adds some more info I normally use for debugging, and of course you can edit it to include more info if you wish:
#define ASSERT(x) \
do { if (x) break; \
printk(KERN_EMERG "### ASSERTION FAILED %s: %s: %d: %s\n", \
__FILE__, __func__, __LINE__, #x); dump_stack(); BUG(); \
} while (0)
BUG_ON() is the appropriate approach to do it. It checks for the condition to be true and calls the macro BUG().
How BUG() handles the rest is explained very well in the following article:
http://kernelnewbies.org/FAQ/BUG
Related
I tried adding this inside the brk system call function :
void *addr = sbrk(0);
printk("current-add-is-%p-\n", addr);
But it returned error during kernel compilation that implicit declaration of sbrk function. And I could not find where sbrk is defined!!
All I need to measure that whenever some user process tries to extended its program break address, I would know its current program break address, so that I can measure how much memory processes are requesting.
Thank you.
Looks like you are trying to do something wrong.
There is no 'sbrk' syscall, there is 'brk'. Except then it would be named sys_brk, but you have no reasons to call it. So if you want to find out how to learn the current break address, read brk's sources.
However, where exactly did you put this in if you did not happen to find brk's sources?
Add this line of code:
printf("Address of program break is %p\n", (void *)sbrk(0));
It will return a message to terminal with hex address of the program break.(e.g., 0x#### #### ####.)
If you want the address in other than hex, then use %u or similar. The use of sbrk(0) is documented in man pages (linux programmers manual).
To see documentation, type in command line: man sbrk and documentation will pop up.
I have a question about the filp_open function:
I can get the error number from the IS_ERR function but I do not understand the meaning of the error number.
Where can find the filp_open error number definitions?
You should not use filp_open to read/write files in kernel mode. For (obvious) security reasons. Other reasons can be found in this answer and this answer (taken from this comment). The official documentation also recomends not to use flp_open:
This is the helper to open a file from kernelspace if you really have to. But in generally you should not do this, so please move along, nothing to see here..
Error code definitions
The kernel uses the same error numbers (errno) in kernel space as in the user space. So, as OmnipotentEntity pointed out, you can see man errno for a reference on what the errors generally mean.
It is also helpful to have a look at the actual implementation of filp_open and its possible error sources, such as file_open_name and build_open_flags.
Note that IS_ERR does not return the error but merely returns whether the supplied pointer is an error value or not. You have to use PTR_ERR to retrieve the error value from the pointer in case IS_ERR is true. Example:
fptr = filp_open(...)
if (IS_ERR(fptr)) {
printk("%d\n", PTR_ERR(fptr));
}
Given an untrusted memory address, is there a way in Linux to test whether it points to valid, accessible memory?
For example, in mach you can use vm_read_overwrite() to attempt to copy data from the specified location. If the address is invalid or inaccessible, it will return an error code rather than crashing the process.
write from that memory (into /dev/null, for example (EDIT: with /dev/null it might not work as expected, use a pipe)), and you'll receive EFAULT error if the address is unaccessible.
I have no idea how to test for writable memory without destroying its content if it is writable.
This a typical case of TOCTOU - you check at some point that the memory is writeable, then later on you try to write to it, and somehow (e.g. because the application deallocated it), the memory is no longer accessible.
There is only one valid way to actually do this, and that is, trap the fault you get from writing to it when you actually need to use it.
Of course, you can use tricks to try to figure out if the memory "may be writeable", but there is no way you can actually ensure it is writeable.
You may want to explain slightly more what you are actually trying to do, and maybe we can have some better ideas if you are more specific.
You can try msync:
int page_size = getpagesize();
void *aligned = (void *)((uintptr_t)p & ~(page_size - 1));
if (msync(aligned, page_size, MS_ASYNC) == -1 && errno == ENOMEM) {
// Non-accessibe
}
But this function may be slow and should not be used in performance critical circumstance.
I am new to kernel programming and I have two questions:
My device is getting registered (by dynamic registration) but my
application is not able to open the device file. What could be the
possible reasons?
What would be the appropriate error code to return when my device
driver detects an divide by zero?
My code implements simple arithmetic operations in the kernel. I use an ioctl() based interface to communicate between user space and the kernel.
if(out.b==0) /*checking for divide by zero*/
out.res=-EINVAL;
else
out.res=out.a/out.b;
copy_to_user((values*)ioctl_param,&out,sizeof(values));
break;
We can't possibly answer the first question if you don't show us your code.
As for the second, EINVAL or perhaps ERANGE.
In your case you need to make the distinction between the information you return in the ioctl_param structure (that's a really bad variable name by the way) and the return status of the ioctl() call itself.
Remember that ioctl() returns 0 if it completes successfully, and sets errno if it fails.
The kernel and C library take care of most of that for you. Usually all you have to do is return -EINVAL or similar from your ioctl() function.
Something like this:
if(out.b == 0) /*checking for divide by zero*/
return -EINVAL;
out.res=out.a / out.b;
copy_to_user((values*)ioctl_param,&out,sizeof(values));
break;
Back story: While running a program under strace I notice that '/dev/urandom' is being open'ed. I would like to know where this call is coming from (it is not part of the program itself, it is part of the system).
So, using gdb, I am trying to break (using catch syscall open) program execution when the open call is issued, so I can see a backtrace. The problem is that open is being called alot, like several hundred times so I can't narrow down the specific call that is opening /dev/urandom. How should I go about narrowing down the specific call? Is there a way to filter by arguments, and if so how do I do it for a syscall?
Any advice would be helpful -- maybe I am going about this all wrong.
GDB is a pretty powerful tool, but has a bit of a learning curve.
Basically, you want to set up a conditional breakpoint.
First use the -i flag to strace or objdump -d to find the address of the open function or more realistically something in the chain of getting there, such as in the plt.
set a breakpoint at that address (if you have debug symbols, you can use those instead, omitting the *, but I'm assuming you don't - though you may well have them for library functions if nothing else.
break * 0x080482c8
Next you need to make it conditional
(Ideally you could compare a string argument to a desired string. I wasn't getting this to work within the first few minutes of trying)
Let's hope we can assume the string is a constant somewhere in the program or one of the libraries it loads. You could look in /proc/pid/maps to get an idea of what is loaded and where, then use grep to verify the string is actually in a file, objdump -s to find it's address, and gdb to verify that you've actually found it in memory by combining the high part of the address from maps with the low part from the file. (EDIT: it's probably easier to use ldd on the executable than look in /proc/pid/maps)
Next you will need to know something about the abi of the platform you are working on, specifically how arguments are passed. I've been working on arm's lately, and that's very nice as the first few arguments just go in registers r0, r1, r2... etc. x86 is a bit less convenient - it seems they go on the stack, ie, *($esp+4), *($esp+8), *($esp+12).
So let's assume we are on an x86, and we want to check that the first argument in esp+4 equals the address we found for the constant we are trying to catch it passing. Only, esp+4 is a pointer to a char pointer. So we need to dereference it for comparison.
cond 1 *(char **)($esp+4)==0x8048514
Then you can type run and hope for the best
If you catch your breakpoint condition, and looking around with info registers and the x command to examine memory seems right, then you can use the return command to percolate back up the call stack until you find something you recognize.
(Adapted from a question edit)
Following Chris's answer, here is the process that eventually got me what I was looking for:
(I am trying to find what functions are calling the open syscall on "/dev/urandom")
use ldd on executable to find loaded libraries
grep through each lib (shell command) looking for 'urandom'
open library file in hex editor and find address of string
find out how parameters are passed in syscalls (for open, file is first parameter. on x86_64 it is passed in rdi -- your mileage may vary
now we can set the conditional breakpoint: break open if $rdi == _addr_
run program and wait for break to hit
run bt to see backtrace
After all this I find that glib's g_random_int() and g_rand_new() use urandom. Gtk+ and ORBit were calling these functions -- if anybody was curious.
Like Andre Puel said:
break open if strcmp($rdi,"/dev/urandom") == 0
Might do the job.