Messages in this thread | | | From | Sumit Garg <> | Date | Wed, 6 Oct 2021 11:05:52 +0530 | Subject | Re: [PATCH v2] arm64: ftrace: use function_nocfi for _mcount as well |
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Hi Mark, Sami,
On Tue, 5 Oct 2021 at 21:05, Mark Rutland <mark.rutland@arm.com> wrote: > > 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. >
Ah, I see.
> Sumit, have you actually seen a problem, or was this found by > inspection? >
Actually I have seen this ftrace problem with the android11-5.4-lts kernel and AOSP master user-space on db845c. The reason being kernel v5.4 LTS doesn't support ftrace with -fpatchable-function-entry on arm64.
With the mainline, I haven't tried to reproduce this issue but it was rather by inspection that this needs to be fixed as well.
> 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.
Sure, let me drop the fixes tag and update the commit description in v3 as mainline only enabled CFI_CLANG for arm64 when "-fpatchable-function-entry" is supported.
-Sumit
> > Thanks, > Mark.
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