Messages in this thread Patch in this message | | | Date | Thu, 8 Apr 2021 20:12:55 +0200 | From | Greg KH <> | Subject | Re: [PATCH] export: Make CRCs robust to symbol trimming |
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On Thu, Apr 08, 2021 at 06:01:05PM +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> > --- > include/linux/export.h | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/include/linux/export.h b/include/linux/export.h > index 6271a5d9c988..27d848712b90 100644 > --- a/include/linux/export.h > +++ b/include/linux/export.h > @@ -140,7 +140,12 @@ struct kernel_symbol { > #define ___cond_export_sym(sym, sec, ns, enabled) \ > __cond_export_sym_##enabled(sym, sec, ns) > #define __cond_export_sym_1(sym, sec, ns) ___EXPORT_SYMBOL(sym, sec, ns) > + > +#ifdef __GENKSYMS__ > +#define __cond_export_sym_0(sym, sec, ns) __GENKSYMS_EXPORT_SYMBOL(sym) > +#else > #define __cond_export_sym_0(sym, sec, ns) /* nothing */ > +#endif > > #else >
Anything to help make these symbol values more "stable" is good, they drive me crazy...
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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