Messages in this thread | | | Date | Tue, 5 Oct 2021 16:35:23 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2] arm64: ftrace: use function_nocfi for _mcount as well |
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On Tue, Oct 05, 2021 at 08:20:02AM -0700, Sami Tolvanen wrote: > Hi Sumit, > > On Tue, Oct 5, 2021 at 5:37 AM Sumit Garg <sumit.garg@linaro.org> wrote: > > > > Commit 800618f955a9 ("arm64: ftrace: use function_nocfi for ftrace_call") > > only fixed address of ftrace_call but address of _mcount needs to be > > fixed as well. Use function_nocfi() to get the actual address of _mcount > > function as with CONFIG_CFI_CLANG, the compiler replaces function pointers > > with jump table addresses which breaks dynamic ftrace as the address of > > _mcount is replaced with the address of _mcount.cfi_jt. > > > > This problem won't apply where the toolchain implements > > -fpatchable-function-entry as we'll use that in preference to regular -pg, > > i.e. this won't show up with recent versions of clang. > > > > Fixes: 9186ad8e66bab6a1 ("arm64: allow CONFIG_CFI_CLANG to be selected") > > Signed-off-by: Sumit Garg <sumit.garg@linaro.org> > > Acked-by: Mark Rutland <mark.rutland@arm.com> > > --- > > > > Changes in v2: > > - Added fixes tag. > > - Extended commit description. > > - Picked up Mark's ack. > > > > arch/arm64/include/asm/ftrace.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h > > index 91fa4baa1a93..347b0cc68f07 100644 > > --- a/arch/arm64/include/asm/ftrace.h > > +++ b/arch/arm64/include/asm/ftrace.h > > @@ -15,7 +15,7 @@ > > #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS > > #define ARCH_SUPPORTS_FTRACE_OPS 1 > > #else > > -#define MCOUNT_ADDR ((unsigned long)_mcount) > > +#define MCOUNT_ADDR ((unsigned long)function_nocfi(_mcount)) > > #endif > > > > /* The BL at the callsite's adjusted rec->ip */ > > -- > > 2.17.1 > > > > Clang >= 10 supports -fpatchable-function-entry and CFI requires Clang > 12, so I assume this is only an issue if > CONFIG_DYNAMIC_FTRACE_WITH_REGS is explicitly disabled?
I don't believe it's possible to disable explicitly, since DYNAMIC_FTRACE_WITH_REGS isn't user selectable, and is def bool y, depending on HAVE_DYNAMIC_FTRACE_WITH_REGS.
Sumit, have you actually seen a problem, or was this found by inspection?
If this isn't an issue in practice, we could add the funciton_nocfi() for consistency, but we should make that clear in the commit message, and drop the fixes tag.
Thanks, Mark.
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