[PATCH 3/4] alpha: fix a 14 years old bug in sigreturn tracing
From: Al Viro
Date: Sat Sep 18 2010 - 04:41:21 EST
The way sigreturn() is implemented on alpha breaks PTRACE_SYSCALL,
all way back to 1.3.95 when alpha has grown PTRACE_SYSCALL support.
What happens is direct return to ret_from_syscall, in order to bypass
mangling of a3 (error indicator) and prevent other mutilations of
registers (e.g. by syscall restart). That's fine, but... the entire
TIF_SYSCALL_TRACE codepath is kept separate on alpha and post-syscall
stopping/notifying the tracer is after the syscall. And the normal
path we are forcibly switching to doesn't have it.
So we end up with *one* stop in traced sigreturn() vs. two in other
syscalls. And yes, strace is visibly broken by that; try to strace
the following
#include <signal.h>
#include <stdio.h>
void f(int sig) {}
main()
{
signal(SIGHUP, f);
raise(SIGHUP);
write(1, "eeeek\n", 6);
}
and watch the show. The
close(1) = 405
in the end of strace output is coming from return value of write() (6 ==
__NR_close on alpha) and syscall number of exit_group() (__NR_exit_group ==
405 there).
The fix is fairly simple - the only thing we end up missing is the call
of syscall_trace() and we can tell whether we'd been called from the
SYSCALL_TRACE path by checking ra value. Since we are setting the
switch_stack up (that's what sys_sigreturn() does), we have the right
environment for calling syscall_trace() - just before we call
undo_switch_stack() and return. Since undo_switch_stack() will overwrite
s0 anyway, we can use it to store the result of "has it been called from
SYSCALL_TRACE path?" check. The same thing applies in rt_sigreturn().
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
arch/alpha/kernel/entry.S | 13 +++++++++++--
1 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/arch/alpha/kernel/entry.S b/arch/alpha/kernel/entry.S
index bceb325..a3e9cd8 100644
--- a/arch/alpha/kernel/entry.S
+++ b/arch/alpha/kernel/entry.S
@@ -430,6 +430,7 @@ strace:
beq $1, 1f
ldq $27, 0($2)
1: jsr $26, ($27), sys_gettimeofday
+ret_from_straced:
ldgp $gp, 0($26)
/* check return.. */
@@ -757,11 +758,15 @@ sys_vfork:
.ent sys_sigreturn
sys_sigreturn:
.prologue 0
+ lda $9, ret_from_straced
+ cmpult $26, $9, $9
mov $sp, $17
lda $18, -SWITCH_STACK_SIZE($sp)
lda $sp, -SWITCH_STACK_SIZE($sp)
jsr $26, do_sigreturn
- br $1, undo_switch_stack
+ bne $9, 1f
+ jsr $26, syscall_trace
+1: br $1, undo_switch_stack
br ret_from_sys_call
.end sys_sigreturn
@@ -770,11 +775,15 @@ sys_sigreturn:
.ent sys_rt_sigreturn
sys_rt_sigreturn:
.prologue 0
+ lda $9, ret_from_straced
+ cmpult $26, $9, $9
mov $sp, $17
lda $18, -SWITCH_STACK_SIZE($sp)
lda $sp, -SWITCH_STACK_SIZE($sp)
jsr $26, do_rt_sigreturn
- br $1, undo_switch_stack
+ bne $9, 1f
+ jsr $26, syscall_trace
+1: br $1, undo_switch_stack
br ret_from_sys_call
.end sys_rt_sigreturn
--
1.5.6.5
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