Messages in this thread |  | | From | Alviro Iskandar Setiawan <> | Date | Sun, 20 Mar 2022 17:33:23 +0700 | Subject | Re: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments |
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On Sun, Mar 20, 2022 at 4:37 PM Ammar Faizi wrote: > In i386, the 6th argument of syscall goes in %ebp. However, both Clang > and GCC cannot use %ebp in the clobber list and in the "r" constraint > without using -fomit-frame-pointer. To make it always available for any > kind of compilation, the below workaround is implemented. > > For clang (the Assembly statement can't clobber %ebp): > 1) Save the %ebp value to the redzone area -4(%esp). > 2) Load the 6-th argument from memory to %ebp. > 3) Subtract the %esp by 4. > 4) Do the syscall (int $0x80). > 5) Pop %ebp.
I don't think you can safely use redzone from inline Assembly. The compiler may also use redzone for a leaf function. In case the syscall is done at the same time, your %ebp saving will clobber the redzone that the compiler uses.
> For GCC, fortunately it has a #pragma that can force a specific function > to be compiled with -fomit-frame-pointer, so it can always use "r"(var) > where `var` is a variable bound to %ebp. > > Cc: x86@kernel.org > Cc: llvm@lists.linux.dev > Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org> [...] > +#if defined(__clang__) > +static inline long ____do_syscall6(long eax, long ebx, long ecx, long edx, > + long esi, long edi, long ebp) > +{ > + __asm__ volatile ( > + "movl %%ebp, -4(%%esp)\n\t" > + "movl %[arg6], %%ebp\n\t" > + "subl $4, %%esp\n\t" > + "int $0x80\n\t" > + "popl %%ebp\n\t" > + : "=a"(eax) > + : "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi), > + [arg6]"m"(ebp) > + : "memory", "cc" > + ); > + return eax; > +} > +
-4(%esp) may be used by the compiler on a leaf call, you can't clobber that.
-- Viro
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