linux-kernel - suspicious compiler warning in vdso2c.h

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Date:	Tue, 16 Sep 2014 10:53:50 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andy Lutomirski" <luto@...capital.net>
Cc:	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: suspicious compiler warning in vdso2c.h

Andy

following e6577a7ce9 ("x86, vdso: Move the vvar area before the
vdso text") gcc 4.3.4 tells me

.../arch/x86/vdso/vdso2c.c: In function ‘main’:
.../arch/x86/vdso/vdso2c.h:118: warning: assuming signed overflow does not occur when assuming that (X + c) < X is
always false
.../arch/x86/vdso/vdso2c.h:118: warning: assuming signed overflow does not occur when assuming that (X + c) < X is
always false

apparently because it unrolls the containing loop and finds i ==
sym_vvar_start on the first iteration. I think there was a hidden
dependency before your change on required_syms[] first two
entries matching up with special_pages[]. If that's correct, all
syms[i] within said loop would seem to need replacing by
syms[special_pages[i]] (similarly for required_syms[i] afaict, see
below draft patch), which also is in line with the loop iterating
over the array elements of special_pages[].

Question then of course is - are there any further hidden
dependencies that already got or may eventually get broken?

Jan

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

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