On Wed, Jan 03, 2018 at 11:39:27PM +0100, Arnd Bergmann wrote: > While testing other changes, I discovered that gcc-7.2.1 produces badly > optimized code for aes_encrypt/aes_decrypt. This is especially true when > CONFIG_UBSAN_SANITIZE_ALL is enabled, where it leads to extremely > large stack usage that in turn might cause kernel stack overflows: > > crypto/aes_generic.c: In function 'aes_encrypt': > crypto/aes_generic.c:1371:1: warning: the frame size of 4880 bytes is larger than 2048 bytes [-Wframe-larger-than=] > crypto/aes_generic.c: In function 'aes_decrypt': > crypto/aes_generic.c:1441:1: warning: the frame size of 4864 bytes is larger than 2048 bytes [-Wframe-larger-than=] > > I verified that this problem exists on all architectures that are > supported by gcc-7.2, though arm64 in particular is less affected than > the others. I also found that gcc-7.1 and gcc-8 do not show the extreme > stack usage but still produce worse code than earlier versions for this > file, apparently because of optimization passes that generally provide > a substantial improvement in object code quality but understandably fail > to find any shortcuts in the AES algorithm. > > Possible workarounds include > > a) disabling -ftree-pre and -ftree-sra optimizations, this was an earlier > patch I tried, which reliably fixed the stack usage, but caused a > serious performance regression in some versions, as later testing > found. > > b) disabling UBSAN on this file or all ciphers, as suggested by Ard > Biesheuvel. This would lead to massively better crypto performance in > UBSAN-enabled kernels and avoid the stack usage, but there is a concern > over whether we should exclude arbitrary files from UBSAN at all. > > c) Forcing the optimization level in a different way. Similar to a), > but rather than deselecting specific optimization stages, > this now uses "gcc -Os" for this file, regardless of the > CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE/SIZE option. This is a reliable > workaround for the stack consumption on all architecture, and I've > retested the performance results now on x86, cycles/byte (lower is > better) for cbc(aes-generic) with 256 bit keys: > > -O2 -Os > gcc-6.3.1 14.9 15.1 > gcc-7.0.1 14.7 15.3 > gcc-7.1.1 15.3 14.7 > gcc-7.2.1 16.8 15.9 > gcc-8.0.0 15.5 15.6 > > This implements the option c) by enabling forcing -Os on all compiler > versions starting with gcc-7.1. As a workaround for PR83356, it would > only be needed for gcc-7.2+ with UBSAN enabled, but since it also shows > better performance on gcc-7.1 without UBSAN, it seems appropriate to > use the faster version here as well. > > Side note: during testing, I also played with the AES code in libressl, > which had a similar performance regression from gcc-6 to gcc-7.2, > but was three times slower overall. It might be interesting to > investigate that further and possibly port the Linux implementation > into that. > > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83356 > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83651 > Cc: Richard Biener <rguenther@xxxxxxx> > Cc: Jakub Jelinek <jakub@xxxxxxxxxxx> > Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> Patch applied. Thanks. -- Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
