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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"); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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