Messages in this thread |  | | From | David Laight <> | Subject | RE: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments | Date | Sun, 20 Mar 2022 13:10:33 +0000 |
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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.
David
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