Re: [PATCH] binfmt_elf: dynamically allocate note.data in parse_elf_properties

[Christian Marangi <ansuelsmth@gmail.com>]

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)

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?

-- 
	Ansuel