X-Git-Url: http://git.onelab.eu/?a=blobdiff_plain;f=arch%2Fi386%2Fkernel%2Fvsyscall-sysenter.S;h=1a36d26e15eb0c6d370832ee166dd0fba3700f59;hb=97bf2856c6014879bd04983a3e9dfcdac1e7fe85;hp=4daefb2ec1b281fb5e9169e2440f54cb840d2354;hpb=6a77f38946aaee1cd85eeec6cf4229b204c15071;p=linux-2.6.git

diff --git a/arch/i386/kernel/vsyscall-sysenter.S b/arch/i386/kernel/vsyscall-sysenter.S
index 4daefb2ec..1a36d26e1 100644
--- a/arch/i386/kernel/vsyscall-sysenter.S
+++ b/arch/i386/kernel/vsyscall-sysenter.S
@@ -7,6 +7,24 @@
  *    for details.
  */
 
+/*
+ * The caller puts arg2 in %ecx, which gets pushed. The kernel will use
+ * %ecx itself for arg2. The pushing is because the sysexit instruction
+ * (found in entry.S) requires that we clobber %ecx with the desired %esp.
+ * User code might expect that %ecx is unclobbered though, as it would be
+ * for returning via the iret instruction, so we must push and pop.
+ *
+ * The caller puts arg3 in %edx, which the sysexit instruction requires
+ * for %eip. Thus, exactly as for arg2, we must push and pop.
+ *
+ * Arg6 is different. The caller puts arg6 in %ebp. Since the sysenter
+ * instruction clobbers %esp, the user's %esp won't even survive entry
+ * into the kernel. We store %esp in %ebp. Code in entry.S must fetch
+ * arg6 from the stack.
+ *
+ * You can not use this vsyscall for the clone() syscall because the
+ * three dwords on the parent stack do not get copied to the child.
+ */
 	.text
 	.globl __kernel_vsyscall
 	.type __kernel_vsyscall,@function
@@ -24,10 +42,10 @@ __kernel_vsyscall:
 	/* 7: align return point with nop's to make disassembly easier */
 	.space 7,0x90
 
-	/* 14: System call restart point is here! (SYSENTER_RETURN - 2) */
+	/* 14: System call restart point is here! (SYSENTER_RETURN-2) */
 	jmp .Lenter_kernel
 	/* 16: System call normal return point is here! */
-	.globl SYSENTER_RETURN	/* Symbol used by entry.S.  */
+	.globl SYSENTER_RETURN	/* Symbol used by sysenter.c  */
 SYSENTER_RETURN:
 	pop %ebp
 .Lpop_ebp: