[1/2] x86-64/setcontext: Pop the pointer into %rdx after syscall
Commit Message
To prepare for shadow stack support, pop the pointer into %rdx after
syscall and use %rdx, instead of %rsi, to restore context. There is
no functional change.
Any comments?
H.J.
----
* sysdeps/unix/sysv/linux/x86_64/setcontext.S (__setcontext):
Pop the pointer into %rdx after syscall and use %rdx, instead
of %rsi, to restore context.
---
sysdeps/unix/sysv/linux/x86_64/setcontext.S | 39 +++++++++++++++--------------
1 file changed, 20 insertions(+), 19 deletions(-)
Comments
On 04/26/2018 02:17 PM, H.J. Lu wrote:
> + /* Pop the pointer into RDX which is preserved by the syscall. */
> + popq %rdx
Sorry, the comment is a bit unclear. Why do you need to restore %rdx if
it is preserved by the system call? Maybe it becomes clearer with a
future change?
Thanks,
Florian
On Sun, Apr 29, 2018 at 2:03 PM, Florian Weimer <fweimer@redhat.com> wrote:
> On 04/26/2018 02:17 PM, H.J. Lu wrote:
>>
>> + /* Pop the pointer into RDX which is preserved by the syscall. */
>> + popq %rdx
>
>
> Sorry, the comment is a bit unclear. Why do you need to restore %rdx if it
> is preserved by the system call? Maybe it becomes clearer with a future
> change?
>
Here is the patch to use it:
https://github.com/hjl-tools/glibc/commit/182ce0987aada98b2d606276d4aa4fbd2527031b
On 04/29/2018 05:03 PM, Florian Weimer wrote:
> On 04/26/2018 02:17 PM, H.J. Lu wrote:
>> + /* Pop the pointer into RDX which is preserved by the syscall.
>> */ + popq %rdx
>
> Sorry, the comment is a bit unclear. Why do you need to restore %rdx
> if it is preserved by the system call? Maybe it becomes clearer with
> a future change?
I agree with Florian, there is no reason to mention RDX is preserved.
It doesn't matter here.
The reason you use RDX is because later on you want to call arch_prctl
and need to use RDI + RSI to pass arguments, and so it's simple to
already be using RDX for the register offset loads.
If anything a better comment is:
/* Pop the pointer into RDX. The choice is arbitrary, but leaving
RDI and RSI available for use later can avoid shuffling values. */
Otherwise OK with that comment change.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
@@ -44,21 +44,22 @@ ENTRY(__setcontext)
movl $_NSIG8,%r10d
movl $__NR_rt_sigprocmask, %eax
syscall
- popq %rdi /* Reload %rdi, adjust stack. */
+ /* Pop the pointer into RDX which is preserved by the syscall. */
+ popq %rdx
cfi_adjust_cfa_offset(-8)
cmpq $-4095, %rax /* Check %rax for error. */
jae SYSCALL_ERROR_LABEL /* Jump to error handler if error. */
/* Restore the floating-point context. Not the registers, only the
rest. */
- movq oFPREGS(%rdi), %rcx
+ movq oFPREGS(%rdx), %rcx
fldenv (%rcx)
- ldmxcsr oMXCSR(%rdi)
+ ldmxcsr oMXCSR(%rdx)
/* Load the new stack pointer, the preserved registers and
registers used for passing args. */
- cfi_def_cfa(%rdi, 0)
+ cfi_def_cfa(%rdx, 0)
cfi_offset(%rbx,oRBX)
cfi_offset(%rbp,oRBP)
cfi_offset(%r12,oR12)
@@ -68,27 +69,27 @@ ENTRY(__setcontext)
cfi_offset(%rsp,oRSP)
cfi_offset(%rip,oRIP)
- movq oRSP(%rdi), %rsp
- movq oRBX(%rdi), %rbx
- movq oRBP(%rdi), %rbp
- movq oR12(%rdi), %r12
- movq oR13(%rdi), %r13
- movq oR14(%rdi), %r14
- movq oR15(%rdi), %r15
+ movq oRSP(%rdx), %rsp
+ movq oRBX(%rdx), %rbx
+ movq oRBP(%rdx), %rbp
+ movq oR12(%rdx), %r12
+ movq oR13(%rdx), %r13
+ movq oR14(%rdx), %r14
+ movq oR15(%rdx), %r15
/* The following ret should return to the address set with
getcontext. Therefore push the address on the stack. */
- movq oRIP(%rdi), %rcx
+ movq oRIP(%rdx), %rcx
pushq %rcx
- movq oRSI(%rdi), %rsi
- movq oRDX(%rdi), %rdx
- movq oRCX(%rdi), %rcx
- movq oR8(%rdi), %r8
- movq oR9(%rdi), %r9
+ movq oRSI(%rdx), %rsi
+ movq oRDI(%rdx), %rdi
+ movq oRCX(%rdx), %rcx
+ movq oR8(%rdx), %r8
+ movq oR9(%rdx), %r9
- /* Setup finally %rdi. */
- movq oRDI(%rdi), %rdi
+ /* Setup finally %rdx. */
+ movq oRDX(%rdx), %rdx
/* End FDE here, we fall into another context. */
cfi_endproc