[tip:x86/asm] x86/asm/entry/32: Document the 32-bit SYSENTER " emergency stack" better

[tip-bot for Denys Vlasenko <tipbot@zytor.com>]

Commit-ID:  d828c71fba8922b116b4ec56c3e5bca8c822d5ae
Gitweb:     http://git.kernel.org/tip/d828c71fba8922b116b4ec56c3e5bca8c822d5ae
Author:     Denys Vlasenko <dvlasenk@redhat.com>
AuthorDate: Mon, 9 Mar 2015 15:52:18 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 17 Mar 2015 09:25:29 +0100

x86/asm/entry/32: Document the 32-bit SYSENTER "emergency stack" better

Before the patch, the 'tss_struct::stack' field was not referenced anywhere.

It was used only to set SYSENTER's stack to point after the last byte
of tss_struct, thus the trailing field, stack[64], was used.

But grep would not know it. You can comment it out, compile,
and kernel will even run until an unlucky NMI corrupts
io_bitmap[] (which is also not easily detectable).

This patch changes code so that the purpose and usage of this
field is not mysterious anymore, and can be easily grepped for.

This does change generated code, for a subtle reason:
since tss_struct is ____cacheline_aligned, there happens to be
5 longs of padding at the end. Old code was using the padding
too; new code will strictly use it only for SYSENTER_stack[].

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1425912738-559-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/processor.h | 4 ++--
 arch/x86/kernel/asm-offsets_32.c | 2 +-
 arch/x86/kernel/cpu/common.c     | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 6a5c0ec..5abd9a5 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -291,9 +291,9 @@ struct tss_struct {
 	unsigned long		io_bitmap[IO_BITMAP_LONGS + 1];
 
 	/*
-	 * .. and then another 0x100 bytes for the emergency kernel stack:
+	 * Space for the temporary SYSENTER stack:
 	 */
-	unsigned long		stack[64];
+	unsigned long		SYSENTER_stack[64];
 
 } ____cacheline_aligned;
 
diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
index 3b3b9d3..47703ae 100644
--- a/arch/x86/kernel/asm-offsets_32.c
+++ b/arch/x86/kernel/asm-offsets_32.c
@@ -68,7 +68,7 @@ void foo(void)
 
 	/* Offset from the sysenter stack to tss.sp0 */
 	DEFINE(TSS_sysenter_sp0, offsetof(struct tss_struct, x86_tss.sp0) -
-		 sizeof(struct tss_struct));
+	       offsetofend(struct tss_struct, SYSENTER_stack));
 
 #if defined(CONFIG_LGUEST) || defined(CONFIG_LGUEST_GUEST) || defined(CONFIG_LGUEST_MODULE)
 	BLANK();
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 7634833..7a3dfb1 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -987,7 +987,7 @@ void enable_sep_cpu(void)
 	}
 
 	tss->x86_tss.ss1 = __KERNEL_CS;
-	tss->x86_tss.sp1 = sizeof(struct tss_struct) + (unsigned long) tss;
+	tss->x86_tss.sp1 = (unsigned long)tss + offsetofend(struct tss_struct, SYSENTER_stack);
 	wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
 	wrmsr(MSR_IA32_SYSENTER_ESP, tss->x86_tss.sp1, 0);
 	wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long) ia32_sysenter_target, 0);