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Message-ID: <CAGXu5jJofYYFiKEbNsU+g11x38JFsPg30AHJbH3hArAgr3PBVA@mail.gmail.com> Date: Wed, 7 Mar 2018 15:56:50 -0800 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: "Tobin C. Harding" <me@...in.cc>, Jonathan Corbet <corbet@....net>, Pantelis Antoniou <pantelis.antoniou@...sulko.com>, "Steven Rostedt (VMware)" <rostedt@...dmis.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>, "Gustavo A. R. Silva" <gustavo@...eddedor.com> Subject: Re: [PATCH] vsprintf: Remove accidental VLA usage On Wed, Mar 7, 2018 at 3:40 PM, Andrew Morton <akpm@...ux-foundation.org> wrote: > On Wed, 7 Mar 2018 15:07:14 -0800 Kees Cook <keescook@...omium.org> wrote: > >> The "sym" calculation is actually a fixed size, but since the max() >> macro uses some extensive tricks for safety, it ends up looking like a >> variable size. This replaces max() with a simple max macro which is >> sufficient for the calculation of the array size. >> >> Seen with -Wvla. Fixed as part of the directive to remove all VLAs from >> the kernel: https://lkml.org/lkml/2018/3/7/621 >> >> ... >> >> --- a/lib/vsprintf.c >> +++ b/lib/vsprintf.c >> @@ -744,8 +744,9 @@ char *resource_string(char *buf, char *end, struct resource *res, >> #define FLAG_BUF_SIZE (2 * sizeof(res->flags)) >> #define DECODED_BUF_SIZE sizeof("[mem - 64bit pref window disabled]") >> #define RAW_BUF_SIZE sizeof("[mem - flags 0x]") >> - char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE, >> - 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)]; >> +#define SIMPLE_MAX(x, y) ((x) > (y) ? (x) : (y)) >> + char sym[SIMPLE_MAX(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE, >> + 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)]; >> >> char *p = sym, *pend = sym + sizeof(sym); >> int decode = (fmt[0] == 'R') ? 1 : 0; > > A year from now I'll be receiving an email titled > > [patch] lib/vsprintf.c: use standard max() macro > > won't I? Hopefully a year from now we'll have added -Wvla to the global build flags and no one will be able to try that. :) -Kees -- Kees Cook Pixel Security
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