| Message ID | 20180426121700.GA30766@intel.com |
|---|---|
| State | New |
| Headers | show |
| Series | [1/2] x86-64/setcontext: Pop the pointer into %rdx after syscall | expand |
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>
diff --git a/sysdeps/unix/sysv/linux/x86_64/setcontext.S b/sysdeps/unix/sysv/linux/x86_64/setcontext.S index 4a9b662074..5a1bdd8de6 100644 --- a/sysdeps/unix/sysv/linux/x86_64/setcontext.S +++ b/sysdeps/unix/sysv/linux/x86_64/setcontext.S @@ -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