path: root/src
diff options
authorRich Felker <>2012-10-11 23:09:17 -0400
committerRich Felker <>2012-10-11 23:09:17 -0400
commit216dca82f6948627f55a0a04ab7d63a12213d8ed (patch)
treec197c176c1bb4649024effabce4052bb597a0437 /src
parent964e9f3c4c8776820e98c967f3a411d4ca5075a7 (diff)
comment possibly-confusing i386 vsyscall asm
Diffstat (limited to 'src')
1 files changed, 13 insertions, 1 deletions
diff --git a/src/internal/i386/syscall.s b/src/internal/i386/syscall.s
index 79296ba0..291168c3 100644
--- a/src/internal/i386/syscall.s
+++ b/src/internal/i386/syscall.s
@@ -1,5 +1,11 @@
.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
@@ -22,11 +28,17 @@ __vsyscall:
jz 1f
push %eax
mov 8(%esp),%eax
- ret
+ ret # tail call to kernel vsyscall entry
1: mov 4(%esp),%eax
int $128
+# 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