From: Ammar Faizi
Sent: 20 March 2022 09:38
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).
i386 doesn't have a redzone.
If you get a signal it will trash -4(%sp)
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.
How is that going to work for an inlined functon?
And using xchg is slow - it is always locked.
One possibility might be to do:
push arg6
push %ebp
mov %ebp, 4(%sp)
int 0x80
pop %ebp
add %esp,4
Although I'm not sure you really want to allocate 4k pages
for every malloc() call.
Probably better to write a mini 'libc' that uses sbrk()
and a best fit scan of a linear free list.