[lkml]   [2022]   [Mar]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRE: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments
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)

> 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.
> 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.


Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

 \ /
  Last update: 2022-03-20 14:11    [W:1.266 / U:0.012 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site