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>

1 files changed, 7 insertions, 5 deletions

diff --git a/arch/x86/vdso/vdso2c.h b/arch/x86/vdso/vdso2c.h
index fd57829b30d8..0224987556ce 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);
         }