Ptrace is giving unexpected SIGTRAP (trace bit) in kernel 2.6.13.2
on both x86 and x86_64.
The 8-instruction program below just execve()s itself over and over.
When run under gdb, the first user-visible SIGTRAP is expected due to
the 'int3'. But the second user-visible SIGTRAP is unexpected, as
there is no reason to trap.
Changing the line "nop; int3" to "nop; nop" gives a program that
just spins merrily when run under /bin/bash. But gdb sees a SIGTRAP,
with the $pc pointing after the second 'nop'. When run under strace
(strace gdb ./execve; (gdb) run), the process spins merrily with
no unexpected SIGTRAP.
-----execve.S
#include <asm/unistd.h>
/*
gcc -o execve -nostartfiles -nostdlib execve.S
gdb ./execve
run
p/x $ps
# 0x202
c
p/x $ps
# 0x302 TF (0x100) set, but should not be
*/
_start: .globl _start
nop; int3
popl %ebp # argc
movl (%esp),%ebx # same filename from argv[0]
movl %esp,%ecx # same argv
lea 4(%esp,%ebp,4),%edx # same envp
movl $__NR_execve,%eax # here we go 'round the mulberry bush, ...
int $0x80
-----end of execve.S
Previous history, and translation for x86_64 are at:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=144805#c23
--
John Reiser, [email protected]
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