Why does strace believe this memory is uninitialized when attaching to a process? - linux

Question

I have an extremely simple program that does nothing more than call recvfrom() in a loop. According to its manpage, one of the arguments is a pointer to the length of the address. This address is initialized in the .data section to the integer value 16. I noticed some strange behavior when I attach to the already-running process to trace it which is not present when I trace the process directly (when I start it traced). Scroll to the end of the lines:
# strace -x -s 10 -e trace=recvfrom ./test
recvfrom(3, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"..., 32, 0, {sa_family=AF_INET, sin_port=htons(42134), sin_addr=inet_addr("127.0.0.1")}, [16]) = 32
recvfrom(3, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"..., 32, 0, {sa_family=AF_INET, sin_port=htons(49442), sin_addr=inet_addr("127.0.0.1")}, [16]) = 32
recvfrom(3, ^Cstrace: Process 18909 detached
<detached ...>
# ./test &
# strace -x -s 10 -e trace=recvfrom -p $!
strace: Process 18916 attached
recvfrom(3, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"..., 32, 0, {sa_family=AF_INET, sin_port=htons(50906), sin_addr=inet_addr("127.0.0.1")}, [1999040176->16]) = 32
recvfrom(3, "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"..., 32, 0, {sa_family=AF_INET, sin_port=htons(52956), sin_addr=inet_addr("127.0.0.1")}, [16]) = 32
recvfrom(3, ^Cstrace: Process 18916 detached
<detached ...>
When I trace it directly, the address length argument shows as [16], which makes sense. After all, the address is a pointer to an int of the value 16. However, when I attach to the process and trace it, the very first call shows that it is not initialized, e.g. [1999040176->16]. This happens for the first syscall every time I attach, but all subsequent calls it shows it correctly as [16]. If I detach from the process and re-attach, the first call will show it as having uninitialized memory.
To be brief:
When I run it under strace, the last argument shows [16] for every recvfrom().
When I attach to it when it is already running, the last argument shows things like [1999040176->16] in the first call to recvfrom(), and [16] in all subsequent ones.
If I detach from it and attach again, the first call to recvfrom() again displays this odd behavior, and all subsequent calls display the expected [16].
The program itself is correct. Here is the program (written in MIPS assembly):
.section .text
.global __start
__start:
# socket
li $v0,4183
li $a0,2
li $a1,1
li $a2,0
syscall
sw $v0,sockfd
# bind
li $v0,4169
lw $a0,sockfd
la $a1,sockaddr_b
li $a2,16
syscall
loop:
# recvfrom
li $v0,4176
lw $a0,sockfd
la $a1,buffer
li $a2,32
li $a3,0
la $t0,sockaddr_a
sw $t0,16($sp)
la $t0,addrlen
sw $t0,20($sp)
syscall
j loop
.section .bss
sockaddr_a: .space 16
buffer: .space 32
sockfd: .space 4
.section .data
addrlen: .int 16
.section .rodata
sockaddr_b: .hword 2,1234,0,0