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comment possibly-confusing i386 vsyscall asm

This commit is contained in:
Rich Felker 2012-10-11 23:09:17 -04:00
parent 964e9f3c4c
commit 216dca82f6
1 changed files with 13 additions and 1 deletions
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14
src/internal/i386/syscall.s
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@ -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 .global __vsyscall
.type __vsyscall,@function .type __vsyscall,@function
__vsyscall: __vsyscall:
@ -22,11 +28,17 @@ __vsyscall:
jz 1f jz 1f
push %eax push %eax
mov 8(%esp),%eax mov 8(%esp),%eax
ret ret # tail call to kernel vsyscall entry
1: mov 4(%esp),%eax 1: mov 4(%esp),%eax
int $128 int $128
ret 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 .global __vsyscall6
.type __vsyscall6,@function .type __vsyscall6,@function
__vsyscall6: __vsyscall6: