[tip:x86/vdso] x86/vdso: Fix vdso2c's special_pages[] error checking

[tip:x86/vdso] x86/vdso: Fix vdso2c's special_pages[] error checking

[tip-bot for Andy Lutomirski]

Commit-ID:  f12c1f9002d27374fd205f6e692891116ca22272
Gitweb:     http://git.kernel.org/tip/f12c1f9002d27374fd205f6e692891116ca22272
Author:     Andy Lutomirski <luto@amacapital.net>
AuthorDate: Wed, 30 Jul 2014 14:59:49 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 24 Sep 2014 09:55:38 +0200

x86/vdso: Fix vdso2c's special_pages[] error checking

Stephen Rothwell's compiler did something amazing: it unrolled a
loop, discovered that one iteration of that loop contained an
always-true test, and emitted a warning that will IMO only serve
to convince people to disable the warning.

That bogus warning caused me to wonder what prompted such an
absurdity from his compiler, and I discovered that the code in
question was, in fact, completely wrong -- I was looking things
up in the wrong array.

This affects 3.16 as well, but the only effect is to screw up
the error checking a bit.  vdso2c's output is unaffected.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/53d96ad5.80ywqrbs33ZBCQej%25akpm@linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/vdso/vdso2c.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index fd57829..0224987 100644
--- a/arch/x86/vdso/vdso2c.h
+++ b/arch/x86/vdso/vdso2c.h
@@ -109,16 +109,18 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
 
 	/* Validate mapping addresses. */
 	for (i = 0; i < sizeof(special_pages) / sizeof(special_pages[0]); i++) {
-		if (!syms[i])
+		INT_BITS symval = syms[special_pages[i]];
+
+		if (!symval)
 			continue;  /* The mapping isn't used; ignore it. */
 
-		if (syms[i] % 4096)
+		if (symval % 4096)
 			fail("%s must be a multiple of 4096\n",
 			     required_syms[i].name);
-		if (syms[sym_vvar_start] > syms[i] + 4096)
-			fail("%s underruns begin_vvar\n",
+		if (symval + 4096 < syms[sym_vvar_start])
+			fail("%s underruns vvar_start\n",
 			     required_syms[i].name);
-		if (syms[i] + 4096 > 0)
+		if (symval + 4096 > 0)
 			fail("%s is on the wrong side of the vdso text\n",
 			     required_syms[i].name);
 	}