linux-kernel - [PATCH 3/9] x86/asm/entry/64: do not SAVE_EXTRA_REGS in stub

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Date:	Thu,  2 Apr 2015 16:36:12 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/9] x86/asm/entry/64: do not SAVE_EXTRA_REGS in stub_sigreturn

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@...hat.com>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Borislav Petkov <bp@...en8.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...capital.net>
CC: Oleg Nesterov <oleg@...hat.com>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: Alexei Starovoitov <ast@...mgrid.com>
CC: Will Drewry <wad@...omium.org>
CC: Kees Cook <keescook@...omium.org>
CC: x86@...nel.org
CC: linux-kernel@...r.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

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