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.
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: [email protected]
Cc: [email protected]
Signed-off-by: Ammar Faizi <[email protected]>
---
tools/include/nolibc/arch-i386.h | 64 ++++++++++++++++++++++++++++++++
1 file changed, 64 insertions(+)
diff --git a/tools/include/nolibc/arch-i386.h b/tools/include/nolibc/arch-i386.h
index 82bf797849ae..10de54d4b4d6 100644
--- a/tools/include/nolibc/arch-i386.h
+++ b/tools/include/nolibc/arch-i386.h
@@ -167,6 +167,70 @@ struct sys_stat_struct {
_ret; \
})
+
+/*
+ * 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.
+ *
+ * For GCC, fortunately it has a #pragma that can force a specific function
+ * to be compiled with -fomit-frame-pointer, so it can use "r"(var) where
+ * var is a variable bound to %ebp.
+ *
+ */
+#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;
+}
+
+#else /* #if defined(__clang__) */
+#pragma GCC push_options
+#pragma GCC optimize "-fomit-frame-pointer"
+static inline long ____do_syscall6(long eax, long ebx, long ecx, long edx,
+ long esi, long edi, long ebp)
+{
+ register long __ebp __asm__("ebp") = ebp;
+ __asm__ volatile (
+ "int $0x80"
+ : "=a"(eax)
+ : "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi),
+ "r"(__ebp)
+ : "memory", "cc"
+ );
+ return eax;
+}
+#pragma GCC pop_options
+#endif /* #if defined(__clang__) */
+
+#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) ( \
+ ____do_syscall6((long)(num), (long)(arg1), \
+ (long)(arg2), (long)(arg3), \
+ (long)(arg4), (long)(arg5), \
+ (long)(arg6)) \
+)
+
+
/* startup code */
/*
* i386 System V ABI mandates:
--
Ammar Faizi
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: [email protected]
> Cc: [email protected]
> Signed-off-by: Ammar Faizi <[email protected]>
[...]
> +#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
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: [email protected]
> > Cc: [email protected]
> > Signed-off-by: Ammar Faizi <[email protected]>
> [...]
> > +#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
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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