On Wed, Jun 07, 2023 at 08:31:58PM +0200, Christian Marangi wrote: > On Wed, Jun 07, 2023 at 02:19:51PM -0700, Kees Cook wrote: > > On Wed, Jun 07, 2023 at 04:42:27PM +0200, Christian Marangi wrote: > > > Dynamically allocate note.data in parse_elf_properties to fix > > > compilation warning on some arch. > > > > I'd rather avoid dynamic allocation as much as possible in the exec > > path, but we can balance it against how much it may happen. > > > > I guess there isn't a good way to handle this other than static global > variables and kmalloc. But check the arch question for additional info > on the case. > > > > On some arch note.data exceed the stack limit for a single function and > > > this cause the following compilation warning: > > > fs/binfmt_elf.c: In function 'parse_elf_properties.isra': > > > fs/binfmt_elf.c:821:1: error: the frame size of 1040 bytes is larger than 1024 bytes [-Werror=frame-larger-than=] > > > 821 | } > > > | ^ > > > cc1: all warnings being treated as errors > > > > Which architectures see this warning? > > > > This is funny. On OpenWRT we are enforcing WERROR and we had FRAME_WARN > hardcoded to 1024. (the option is set to 2048 on 64bit arch) Ah-ha. Okay, I was wondering how you got that. :) > ARCH_USE_GNU_PROPERTY is set only on arm64 that have a FRAME_WARN set to > 2048. > > So this was triggered by building arm64 with FRAME_WARN set to 1024. > > Now with the configuration of 2048 the stack warn is not triggered, but > I wonder if it may happen to have a 32bit system with > ARCH_USE_GNU_PROPERTY. That would effectively trigger the warning. > > So this is effectively a patch that fix a currently not possible > configuration, since: > > !IS_ENABLED(CONFIG_ARCH_USE_GNU_PROPERTY) will result in node.data > effectively never allocated by the compiler are the function will return > 0 on everything that doesn't have CONFIG_ARCH_USE_GNU_PROPERTY. > > > > Fix this by dynamically allocating the array. > > > Update the sizeof of the union to the biggest element allocated. > > > > How common are these notes? I assume they're very common; I see them > > even in /bin/true: > > > > $ readelf -lW /bin/true | grep PROP > > GNU_PROPERTY 0x000338 0x0000000000000338 0x0000000000000338 0x000030 0x000030 R 0x8 > > > > -- > > Is there a way to check if this kmalloc actually cause perf regression? I don't have a good benchmark besides just an exec loop. But since this isn't reachable in a regular config, I'd rather keep things how there already are. -Kees -- Kees Cook
