Re: [PATCH] arm64: ftrace: Fix to enable syscall events on arm64

[Will Deacon]

Hi Masami,

On Wed, Nov 28, 2018 at 01:29:45AM +0900, Masami Hiramatsu wrote:
> Since commit 4378a7d4be30 ("arm64: implement syscall wrappers")
> introduced "__arm64_" prefix to all syscall wrapper symbols in
> sys_call_table, syscall tracer can not find corresponding
> metadata from syscall name. In the result, we have no syscall
> ftrace events on arm64 kernel, and some bpf testcases are failed
> on arm64.
> 
> To fix this issue, this introduces custom
> arch_syscall_match_sym_name() which skips first 8 bytes when
> comparing the syscall and symbol names.
> 
> Fixes: 4378a7d4be30 ("arm64: implement syscall wrappers")
> Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/arm64/include/asm/ftrace.h |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index caa955f10e19..a710f79db442 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -56,6 +56,15 @@ static inline bool arch_trace_is_compat_syscall(struct pt_regs *regs)
>  {
>  	return is_compat_task();
>  }
> +
> +#define ARCH_HAS_SYSCALL_MATCH_SYM_NAME
> +
> +static inline bool arch_syscall_match_sym_name(const char *sym,
> +					       const char *name)
> +{
> +	/* Since all syscall functions have __arm64_ prefix, we must skip it */
> +	return !strcmp(sym + 8, name);
> +}

This looks fine to me, but I'm curious about whether this is supposed to
work with compat syscalls as well, where the prefix is "__arm64_compat_".

If we broadly follow the x86 lead, we'd have:

	return (!strncmp(sym, "__arm64_", 8) && !strcmp(sym + 8, name)) ||
		(!strncmp(sym, "__arm64_compat_", 15) && !strcmp(sym + 15, name));

Do we need to handle compat (i.e. 32-bit) tasks here?

Will