[PATCH] correctly handle "kill -TRAP $straced_programs_pid" and int3

Denys Vlasenko dvlasenk at redhat.com
Tue Dec 16 20:25:46 UTC 2008

2008-12-16  Denys Vlasenko  <dvlasenk at redhat.com>

	* defs.h: Add new field "sigtrap80" to struct tcb.
	* strace.c (alloc_tcb): Initialize it.
	(detach, trace): Use it in place of constant SIGTRAP.
	options on newly traced threads; detect these
	options if they are inherited across clone(); detect
	and handle execve's ptrace stop; add paranoia checks
	if real SIGTRAP is seen.

Run tested on vanilla and utrace-based kernels.
Tested with "kill -TRAP $pid":

strace sleep N

strace sh -c "trap 'echo Oi' TRAP; sleep 1; sleep 1; sleep 1; sleep 1"

strace -f -oLOG firefox

None of the above works correctly with unpatched strace,
with patch SIGTRAP is visible in strace and acts
as any other signal.

Testing how we strace trapping instruction:

# cat z.S

_start: .globl _start
        movl $42,%ebx
        movl $1,%eax
        int $0x80

# gcc -nostartfiles -nostdlib -oz z.S

# ./strace ./z
execve("./z", ["./z"], [/* 55 vars */]) = 0
--- SIGTRAP (Trace/breakpoint trap) @ 0 (0) ---
+++ killed by SIGTRAP +++

Unpatched strace:

# strace ./z
execve("./z", ["./z"], [/* 55 vars */]) = 0
write(0, NULL, 0 <unfinished ... exit status 42>


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