Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file

* Dave Martin:

On Tue, Jun 18, 2019 at 09:00:35AM -0700, Yu-cheng Yu wrote:

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On Tue, 2019-06-18 at 18:05 +0200, Florian Weimer wrote:

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* Yu-cheng Yu:

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I assumed that it would also parse the main executable and make
adjustments based on that.

Yes, Linux also looks at the main executable's header, but not its
NT_GNU_PROPERTY_TYPE_0 if there is a loader.

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ld.so can certainly provide whatever the kernel needs.  We need to tweak
the existing loader anyway.

No valid statically-linked binaries exist today, so this is not a
consideration at this point.

So from kernel, we look at only PT_GNU_PROPERTY?

If you don't parse notes/segments in the executable for CET, then yes.
We can put PT_GNU_PROPERTY into the loader.

Thanks!

Would this require the kernel and ld.so to be updated in a particular
order to avoid breakage?  I don't know enough about RHEL to know how
controversial that might be.

There is no official ld.so that will work with the current userspace
interface (in this patch submission).  Upstream glibc needs to be
updated anyway, so yet another change isn't much of an issue.  This is
not a problem; we knew that something like this might happen.

Sure, people need a new binutils with backports for PT_GNU_PROPERTY, but
given that only very few people will build CET binaries with older
binutils, I think that's not a real issue either.

Thanks,
Florian