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

[Joe Simmons-Talbott]

On Mon, Apr 17, 2023 at 08:54:17AM -0700, H.J. Lu wrote:
> 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?
> 

Yes.  I'll have a new version combining the two asm sections into one
like for aarch64 soon.

Joe