Messages in this thread | | | From | Fangrui Song <> | Date | Wed, 4 Jan 2023 09:30:22 -0800 | Subject | Re: kCFI && patchable-function-entry=M,N |
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On Mon, Oct 24, 2022 at 4:24 AM Mark Rutland <mark.rutland@arm.com> wrote: > > On Sat, Oct 22, 2022 at 04:57:18PM +0200, Peter Zijlstra wrote: > > On Fri, Oct 21, 2022 at 04:56:20PM +0100, Mark Rutland wrote: > > > Hi, > > > > > > For arm64, I'd like to use -fatchable-function-entry=M,N (where N > 0), for our > > > ftrace implementation, which instruments *some* but not all functions. > > > Unfortuntately, this doesn't play nicely with -fsanitize=kcfi, as instrumented > > > and non-instrumented functions don't agree on where the type hash should live > > > relative to the function entry point, making them incompatible with one another. > > > AFAICT, there's no mechanism today to get them to agree. > > > > > > Today we use -fatchable-function-entry=2, which happens to avoid this. > > > > > ... but I understand that for x86, folk want the pre-function NOPs to > > > fall-through into the body of the function. > > > > Yep. > > > > > Is there any mechanism today that we could use to solve this, or could we > > > extend clang to have some options to control this behaviour? > > > > So the main pain-point for you is differentiating between function with > > notrace and those without it, right? > > > > That is; suppose you (like x86) globally do: > > -fpatchable-function-entry=4,2 to get a consistent function signature, > > you're up a creek because you use the __patchable_function_entries > > section to drive ftrace and now every function will have it. > > > > So perhaps something like: > > > > -fpatchable-function-entry=N,M,sectionname > > > > would help, then you can have notrace be the same layout, except a > > different section. Eg. something like: > > > > #define notrace __attribute__((patchable_function_entry(4,2,__notrace_function_entries))) > > FWIW, I think that'd work for me, and that was roughly my original proposal on > IRC. My only concern with this approach is code size, since all uninstrumented > functions gain some point less prefix NOPs. > > We can make that slghtly better as: > > #define notrace __attribute__((patchable_function_entry(2,2,__notrace_function_entries))) > > ... since we don't care about placing NOPs *within* the function > > > It does make the whole: CFLAGS_REMOVE_file.o = $(CC_FLAGS_FTRACE) > > a bit of a pain, but I've long favoured removing all that and having > > explitic notrace attributes on all relevant functions. > > > > Then again; perhaps it could be made to work by ensuring CFLAGS starts > > with: > > > > -fpatchable-function-entry=4,2,__notrace_function_entries > > > > and have CC_FLAGS_FTRACE include (and hence override with) > > > > -fpatchable-function-entry=4,2,__ftrace_function_entries > > > > assuming that with duplicate argument the last is effective. > > TBH, it'd be nice to move ftrace to the `CFLAGS_WHATEVER_obj.o := n` approach > the other instrumentation uses, which IIUC would allow us to define different > flags for the two cases (though I'll need to go check that). > > Thanks, > Mark.
Hi Mark and Peter,
Sami asked my opinion (as the main -fpatchable-function-entry= implementer on the llvm-project side) on this extension (-fpatchable-function-entry=4,2,__ftrace_function_entries). I think this is fine.
You may consider bringing this up as a GCC feature request (https://gcc.gnu.org/bugzilla/show_bug.cgi) and CCing the author/the committer of https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=417ca0117a1a9a8aaf5bc5ca530adfd68cb00399 (original -fpatchable-function-entry= support) and the author of https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=c23b5006d3ffeda1a9edf5fd817765a6da3696ca (powerpc64 ELFv2 support). On the feature request, a summary (so that toolchain people don't have to read every message in this thread) will help:)
-- 宋方睿
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