[PATCH v3 1/2] x86_64: Set the syscall register right before doing the syscall.

[H.J. Lu]

On Mon, Apr 17, 2023 at 8:35 AM Joe Simmons-Talbott via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> To make identifying syscalls easier during call tree analysis load the
> syscall number just before performing the syscall.
>
> Compiler optimizations can place quite a few instructions between the
> setting of the syscall number and the syscall instruction.  During call
> tree analysis the number of instructions between the two can lead to
> more difficulty for both tools and humans in properly identifying the
> syscall number.  Having the syscall number set in the prior instruction
> to the syscall instruction makes this task easier and less error prone.
> Being able to reliably identify syscalls made by a given API will make
> it easier to understand and verify the safety and security of glibc.
> ---
>  sysdeps/unix/sysv/linux/x86_64/sysdep.h | 33 +++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
>
> diff --git a/sysdeps/unix/sysv/linux/x86_64/sysdep.h b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
> index cfb51be8c5..800a56723f 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/sysdep.h
> +++ b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
> @@ -250,12 +250,20 @@
>      (long int) resultvar;                                              \
>  })
>
> +#define MSTR_HELPER(x) #x
> +#define MSTR(x) MSTR_HELPER(x)
> +
>  #undef internal_syscall1
>  #define internal_syscall1(number, arg1)                                        \
>  ({                                                                     \
>      unsigned long int resultvar;                                       \
>      TYPEFY (arg1, __arg1) = ARGIFY (arg1);                             \
>      register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;                  \
> +    if (__builtin_constant_p(number))                        \
> +      asm volatile ("movl $" MSTR(number) ", %%eax\n\t" \
> +        : /* no outputs */                                   \
> +        : "i" (number)                                       \
> +               : "eax");                                            \
>      asm volatile (                                                     \
>      "syscall\n\t"                                                      \
>      : "=a" (resultvar)                                                 \
> @@ -272,6 +280,11 @@
>      TYPEFY (arg1, __arg1) = ARGIFY (arg1);                             \
>      register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;                  \
>      register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;                  \
> +    if (__builtin_constant_p(number))                        \
> +      asm volatile ("movl $" MSTR(number) ", %%eax\n\t" \
> +        : /* no outputs */                                   \
> +        : "i" (number)                                       \
> +               : "eax");                                            \
>      asm volatile (                                                     \
>      "syscall\n\t"                                                      \
>      : "=a" (resultvar)                                                 \
> @@ -290,6 +303,11 @@
>      register TYPEFY (arg3, _a3) asm ("rdx") = __arg3;                  \
>      register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;                  \
>      register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;                  \
> +    if (__builtin_constant_p(number))                        \
> +      asm volatile ("movl $" MSTR(number) ", %%eax\n\t" \
> +        : /* no outputs */                                   \
> +        : "i" (number)                                       \
> +               : "eax");                                            \
>      asm volatile (                                                     \
>      "syscall\n\t"                                                      \
>      : "=a" (resultvar)                                                 \
> @@ -310,6 +328,11 @@
>      register TYPEFY (arg3, _a3) asm ("rdx") = __arg3;                  \
>      register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;                  \
>      register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;                  \
> +    if (__builtin_constant_p(number))                        \
> +      asm volatile ("movl $" MSTR(number) ", %%eax\n\t" \
> +        : /* no outputs */                                   \
> +        : "i" (number)                                       \
> +               : "eax");                                            \
>      asm volatile (                                                     \
>      "syscall\n\t"                                                      \
>      : "=a" (resultvar)                                                 \
> @@ -332,6 +355,11 @@
>      register TYPEFY (arg3, _a3) asm ("rdx") = __arg3;                  \
>      register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;                  \
>      register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;                  \
> +    if (__builtin_constant_p(number))                        \
> +      asm volatile ("movl $" MSTR(number) ", %%eax\n\t" \
> +        : /* no outputs */                                   \
> +        : "i" (number)                                       \
> +               : "eax");                                            \
>      asm volatile (                                                     \
>      "syscall\n\t"                                                      \
>      : "=a" (resultvar)                                                 \
> @@ -357,6 +385,11 @@
>      register TYPEFY (arg3, _a3) asm ("rdx") = __arg3;                  \
>      register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;                  \
>      register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;                  \
> +    if (__builtin_constant_p(number))                        \
> +      asm volatile ("movl $" MSTR(number) ", %%eax\n\t" \
> +        : /* no outputs */                                   \
> +        : "i" (number)                                       \
> +               : "eax");                                            \
>      asm volatile (                                                     \
>      "syscall\n\t"                                                      \
>      : "=a" (resultvar)                                                 \
> --
> 2.39.2
>

Won't the compiler load EAX twice when number is a constant?

-- 
H.J.