[PATCH] Revert "x86: signal: change type of paramter for sys_rt_sigreturn()"

[Ingo Molnar]

This reverts commit 4217458dafaa57d8e26a46f5d05ab8c53cf64191.

Justin Madru bisected this commit, it was causing weird Firefox
crashes.

The reason is that GCC mis-optimizes (re-uses) the on-stack parameters of
the calling frame, which corrupts the syscall return pt_regs state and
thus corrupts user-space register state.

So we go back to the slightly less clean but more optimization-safe
method of getting to pt_regs. Also add a comment to explain this.

Resolves: http://bugzilla.kernel.org/show_bug.cgi?id=12505

Reported-and-bisected-by: Justin Madru <jdm64@xxxxxxxxx>
Tested-by: Justin Madru <jdm64@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 arch/x86/include/asm/syscalls.h |    2 +-
 arch/x86/kernel/signal.c        |   11 +++++++++--
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/syscalls.h b/arch/x86/include/asm/syscalls.h
index 9c6797c..c0b0bda 100644
--- a/arch/x86/include/asm/syscalls.h
+++ b/arch/x86/include/asm/syscalls.h
@@ -40,7 +40,7 @@ asmlinkage int sys_sigaction(int, const struct old_sigaction __user *,
 			     struct old_sigaction __user *);
 asmlinkage int sys_sigaltstack(unsigned long);
 asmlinkage unsigned long sys_sigreturn(unsigned long);
-asmlinkage int sys_rt_sigreturn(struct pt_regs);
+asmlinkage int sys_rt_sigreturn(unsigned long);
 
 /* kernel/ioport.c */
 asmlinkage long sys_iopl(unsigned long);
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 89bb766..df0587f 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -632,9 +632,16 @@ badframe:
 }
 
 #ifdef CONFIG_X86_32
-asmlinkage int sys_rt_sigreturn(struct pt_regs regs)
+/*
+ * Note: do not pass in pt_regs directly as with tail-call optimization
+ * GCC will incorrectly stomp on the caller's frame and corrupt user-space
+ * register state:
+ */
+asmlinkage int sys_rt_sigreturn(unsigned long __unused)
 {
-	return do_rt_sigreturn(&regs);
+	struct pt_regs *regs = (struct pt_regs *)&__unused;
+
+	return do_rt_sigreturn(regs);
 }
 #else /* !CONFIG_X86_32 */
 asmlinkage long sys_rt_sigreturn(struct pt_regs *regs)
--
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