Re: [PATCH] export: Make CRCs robust to symbol trimming

[Greg Kroah-Hartman]

On Fri, May 21, 2021 at 10:57:44AM +0100, Lee Jones wrote:
> On Mon, 10 May 2021 at 11:25, Quentin Perret <qperret@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > 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}?

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