[PATCH 3/9] x86/asm/entry/64: do not SAVE_EXTRA_REGS in stub_sigreturn

[Denys Vlasenko <dvlasenk@redhat.com>]

stub_sigreturn ignores old values of pt_regs->REG for all general-purpose
registers, it sets them to values saved on userspace
signal stack.

Which is hardly surprising - it would be a bug if it would use pt_regs->REG.
sigreturn must restore all registers.

Therefore, SAVE_EXTRA_REGS in it ought to be redundant.

It is a leftover from the time SAVE_EXTRA_REGS wasn't only saving registers,
but it also was extending stack to "full" pt_regs.

Delete this SAVE_EXTRA_REGS.

Run-tested.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
 arch/x86/kernel/entry_64.S | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index ec51598..1cf245d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -447,7 +447,12 @@ ENTRY(stub_rt_sigreturn)
 	CFI_STARTPROC
 	addq $8, %rsp
 	DEFAULT_FRAME 0
-	SAVE_EXTRA_REGS
+	/*
+	 * Despite RESTORE_EXTRA_REGS in return_from_stub,
+	 * no need to SAVE_EXTRA_REGS here:
+	 * sys_rt_sigreturn overwrites all general purpose pt_regs->REGs
+	 * on stack, for RESTORE_{EXTRA,C}_REGS to pick them up.
+	 */
 	call sys_rt_sigreturn
 	jmp  return_from_stub
 	CFI_ENDPROC
@@ -458,7 +463,7 @@ ENTRY(stub_x32_rt_sigreturn)
 	CFI_STARTPROC
 	addq $8, %rsp
 	DEFAULT_FRAME 0
-	SAVE_EXTRA_REGS
+	/* No need to SAVE_EXTRA_REGS */
 	call sys32_x32_rt_sigreturn
 	jmp  return_from_stub
 	CFI_ENDPROC
-- 
1.8.1.4