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comment possibly-confusing i386 vsyscall asm
author
Rich Felker
<dalias@aerifal.cx>
Fri, 12 Oct 2012 03:09:17 +0000
(23:09 -0400)
committer
Rich Felker
<dalias@aerifal.cx>
Fri, 12 Oct 2012 03:09:17 +0000
(23:09 -0400)
src/internal/i386/syscall.s
patch
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blob
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diff --git
a/src/internal/i386/syscall.s
b/src/internal/i386/syscall.s
index
79296ba
..
291168c
100644
(file)
--- a/
src/internal/i386/syscall.s
+++ b/
src/internal/i386/syscall.s
@@
-1,5
+1,11
@@
.hidden __sysinfo
.hidden __sysinfo
+# The calling convention for __vsyscall has the syscall number
+# and 5 args arriving as: eax, edx, ecx, edi, esi, 4(%esp).
+# This ensures that the inline asm in the C code never has to touch
+# ebx or ebp (which are unavailable in PIC and frame-pointer-using
+# code, respectively), and optimizes for size/simplicity in the caller.
+
.global __vsyscall
.type __vsyscall,@function
__vsyscall:
.global __vsyscall
.type __vsyscall,@function
__vsyscall:
@@
-22,11
+28,17
@@
__vsyscall:
jz 1f
push %eax
mov 8(%esp),%eax
jz 1f
push %eax
mov 8(%esp),%eax
- ret
+ ret
# tail call to kernel vsyscall entry
1: mov 4(%esp),%eax
int $128
ret
1: mov 4(%esp),%eax
int $128
ret
+# The __vsyscall6 entry point is used only for 6-argument syscalls.
+# Instead of passing the 5th argument on the stack, a pointer to the
+# 5th and 6th arguments is passed. This is ugly, but there are no
+# register constraints the inline asm could use that would make it
+# possible to pass two arguments on the stack.
+
.global __vsyscall6
.type __vsyscall6,@function
__vsyscall6:
.global __vsyscall6
.type __vsyscall6,@function
__vsyscall6: