Date: Thu, 22 Mar 2018 08:01:25 -0700 From: Kees Cook <keescook@...omium.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Al Viro <viro@...iv.linux.org.uk>, Florian Weimer <fweimer@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Randy Dunlap <rdunlap@...radead.org>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Ingo Molnar <mingo@...nel.org>, David Laight <David.Laight@...lab.com>, Ian Abbott <abbotti@....co.uk>, linux-input <linux-input@...r.kernel.org>, linux-btrfs <linux-btrfs@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max() On Tue, Mar 20, 2018 at 4:23 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Sat, Mar 17, 2018 at 1:07 PM, Kees Cook <keescook@...omium.org> wrote: >> >> No luck! :( gcc 4.4 refuses to play along. And, hilariously, not only >> does it not change the complaint about __builtin_choose_expr(), it >> also thinks that's a VLA now. > > Hmm. So thanks to the diseased mind of Martin Uecker, there's a better > test for "__is_constant()": > > /* Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de> */ > #define __is_constant(a) \ > (sizeof(int) == sizeof(*(1 ? ((void*)((a) * 0l)) : (int*)1))) > > that is actually *specified* by the C standard to work, and doesn't > even depend on any gcc extensions. I feel we risk awakening Cthulhu with this. :) > The reason is some really subtle pointer conversion rules, where the > type of the ternary operator will depend on whether one of the > pointers is NULL or not. > > And the definition of NULL, in turn, very much depends on "integer > constant expression that has the value 0". > > Are you willing to do one final try on a generic min/max? Same as my > last patch, but using the above __is_constant() test instead of > __builtin_constant_p? So, this time it's not a catastrophic failure with gcc 4.4. Instead it fails in 11 distinct places: $ grep "first argument to ‘__builtin_choose_expr’ not a constant" log | cut -d: -f1-2 crypto/ablkcipher.c:71 crypto/blkcipher.c:70 crypto/skcipher.c:95 mm/percpu.c:2453 net/ceph/osdmap.c:1545 net/ceph/osdmap.c:1756 net/ceph/osdmap.c:1763 mm/kmemleak.c:1371 mm/kmemleak.c:1403 drivers/infiniband/hw/hfi1/pio_copy.c:421 drivers/infiniband/hw/hfi1/pio_copy.c:547 Seems like it doesn't like void * arguments: mm/percpu.c: void *ptr; ... base = min(ptr, base); mm/kmemleak.c: static void scan_large_block(void *start, void *end) ... next = min(start + MAX_SCAN_SIZE, end); I'll poke a bit more... -Kees -- Kees Cook Pixel Security
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