Messages in this thread |  | | From | Alviro Iskandar Setiawan <> | Date | Sun, 20 Mar 2022 17:42:30 +0700 | Subject | Re: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments |
| |
On Sun, Mar 20, 2022 at 5:33 PM Alviro Iskandar Setiawan wrote: > 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.
Using xchgl to preserve %ebp in the same place where the arg6 is stored in memory is a better solution and doesn't clobber anything.
xchgl %ebp, %[arg6] int $0x80 xchgl %ebp, %[arg6]
-- Viro
|  |