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Subject[PATCH 2/2] x86_64 ptrace vs -ENOSYS
Date

When we're stopped at syscall entry tracing, ptrace can change the %rax
value from -ENOSYS to something else. If no system call is actually made
because the syscall number (now in orig_rax) is bad, then we now always
reset %rax to -ENOSYS again.

This changes it to leave the return value alone after entry tracing.
That way, the %rax value set by ptrace is there to be seen in user mode
(or in syscall exit tracing). This is consistent with what the 32-bit
kernel does.

Signed-off-by: Roland McGrath <roland@redhat.com>
---
arch/x86/kernel/entry_64.S | 8 +++-----
1 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index c20c9e7..556a8df 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -319,19 +319,17 @@ badsys:
/* Do syscall tracing */
tracesys:
SAVE_REST
- movq $-ENOSYS,RAX(%rsp)
+ movq $-ENOSYS,RAX(%rsp) /* ptrace can change this for a bad syscall */
FIXUP_TOP_OF_STACK %rdi
movq %rsp,%rdi
call syscall_trace_enter
LOAD_ARGS ARGOFFSET /* reload args from stack in case ptrace changed it */
RESTORE_REST
cmpq $__NR_syscall_max,%rax
- movq $-ENOSYS,%rcx
- cmova %rcx,%rax
- ja 1f
+ ja int_ret_from_sys_call /* RAX(%rsp) set to -ENOSYS above */
movq %r10,%rcx /* fixup for C */
call *sys_call_table(,%rax,8)
-1: movq %rax,RAX-ARGOFFSET(%rsp)
+ movq %rax,RAX-ARGOFFSET(%rsp)
/* Use IRET because user could have changed frame */

/*

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