Messages in this thread | | | Date | Fri, 21 May 2021 12:02:06 +0100 | From | Lee Jones <> | Subject | Re: [PATCH] export: Make CRCs robust to symbol trimming |
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On Fri, 21 May 2021, Greg Kroah-Hartman wrote:
> On Fri, May 21, 2021 at 10:57:44AM +0100, Lee Jones wrote: > > On Mon, 10 May 2021 at 11:25, Quentin Perret <qperret@google.com> wrote: > > > > > Hi, > > > > > > On Thursday 08 Apr 2021 at 18:01:05 (+0000), Quentin Perret wrote: > > > > The CRC calculation done by genksyms is triggered when the parser hits > > > > EXPORT_SYMBOL*() macros. At this point, genksyms recursively expands the > > > > types, and uses that as the input for the CRC calculation. In the case > > > > of forward-declared structs, the type expands to 'UNKNOWN'. Next, the > > > > result of the expansion of each type is cached, and is re-used when/if > > > > the same type is seen again for another exported symbol in the file. > > > > > > > > Unfortunately, this can cause CRC 'stability' issues when a struct > > > > definition becomes visible in the middle of a C file. For example, let's > > > > assume code with the following pattern: > > > > > > > > struct foo; > > > > > > > > int bar(struct foo *arg) > > > > { > > > > /* Do work ... */ > > > > } > > > > EXPORT_SYMBOL_GPL(bar); > > > > > > > > /* This contains struct foo's definition */ > > > > #include "foo.h" > > > > > > > > int baz(struct foo *arg) > > > > { > > > > /* Do more work ... */ > > > > } > > > > EXPORT_SYMBOL_GPL(baz); > > > > > > > > Here, baz's CRC will be computed using the expansion of struct foo that > > > > was cached after bar's CRC calculation ('UNKOWN' here). But if > > > > EXPORT_SYMBOL_GPL(bar) is removed from the file (because of e.g. symbol > > > > trimming using CONFIG_TRIM_UNUSED_KSYMS), struct foo will be expanded > > > > late, during baz's CRC calculation, which now has visibility over the > > > > full struct definition, hence resulting in a different CRC for baz. > > > > > > > > This can cause annoying issues for distro kernel (such as the Android > > > > Generic Kernel Image) which use CONFIG_UNUSED_KSYMS_WHITELIST. Indeed, > > > > as per the above, adding a symbol to the whitelist can change the CRC of > > > > symbols that are already kept exported. As such, modules built against a > > > > kernel with a trimmed ABI may not load against the same kernel built > > > > with an extended whitelist, even though they are still strictly binary > > > > compatible. While rebuilding the modules would obviously solve the > > > > issue, I believe this classifies as an odd genksyms corner case, and it > > > > gets in the way of kernel updates in the GKI context. > > > > > > > > To work around the issue, make sure to keep issuing the > > > > __GENKSYMS_EXPORT_SYMBOL macros for all trimmed symbols, hence making > > > > the genksyms parsing insensitive to symbol trimming. > > > > > > > > Signed-off-by: Quentin Perret <qperret@google.com> > > > > > > Gentle ping. > > > > > > Is there anything else I should do in this one? > > > > > > > With Greg's Ack and ~6 weeks on the list, you're probably golden. > > > > I *could* pick this up, but seems wrong somehow. > > > > Greg, is this something you're prepared to merge? If not, who's the > > g{uy,al}? > > What does get_maintainer.pl show?
It doesn't [0], which is why I commented in this way. :)
[0]:
Jessica Yu <jeyu@kernel.org> (commit_signer:2/5=40%) Emil Velikov <emil.l.velikov@gmail.com> (commit_signer:2/5=40%) Miroslav Benes <mbenes@suse.cz> (commit_signer:2/5=40%) Nick Desaulniers <ndesaulniers@google.com> (commit_signer:2/5=40%,authored:1/5=20%,added_lines:1/7=14%,removed_lines:1/11=9%) Greg Kroah-Hartman <gregkh@linuxfoundation.org> (commit_signer:1/5=20%) Quentin Perret <qperret@google.com> (authored:1/5=20%,added_lines:5/7=71%) Joe Perches <joe@perches.com> (authored:1/5=20%,added_lines:1/7=14%,removed_lines:1/11=9%) linux-kernel@vger.kernel.org (open list)
-- Lee Jones [李琼斯] Senior Technical Lead - Developer Services Linaro.org │ Open source software for Arm SoCs Follow Linaro: Facebook | Twitter | Blog
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