linux-kernel - Re: [PATCH] tracing/x86: Update syscall trace events to handle new x86 syscall func names

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Date:   Tue, 17 Apr 2018 18:10:29 +0200
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH] tracing/x86: Update syscall trace events to handle new
 x86 syscall func names

Steven,

On Tue, Apr 17, 2018 at 11:54:40AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Arnaldo noticed that the latest kernel is missing the syscall event system
> directory in x86. I bisected it down to d5a00528b58c ("syscalls/core,
> syscalls/x86: Rename struct pt_regs-based sys_*() to __x64_sys_*()").
> 
> The system call trace events are special, as there is only one trace event
> for all system calls (the raw_syscalls). But a macro that wraps the system
> calls creates meta data for them that copies the name to find the system
> call that maps to the system call table (the number). At boot up, it does a
> kallsyms lookup for this mapping. If it does not find a function, then that
> system call is ignored.
> 
> Because the x86 system calls had "__x86_" appended to the "sys" for the

__x64 actually.

> names, they do not match the default compare algorithm. As this was a
> problem for power pc, the algorithm can be overwritten by the architecture.
> The solution is to have x86 have its own algorithm to do the compare and
> this brings back the system call trace events.

this lets me wonder *which* syscall "function" should be included in this
directory. Is it really the stub (__x64_sys_waitid, for example), or better
the sign-extending C function (__se_sys_waitid) which then calls an
internal helper (__do_sys_waitid). An additional advantage:
__se_sys_waitid would be available on x86 and, AFAICS, on all other archs.

> Reported-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> Fixes: d5a00528b58c ("syscalls/core, syscalls/x86: Rename struct pt_regs-based sys_*() to __x64_sys_*()")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  arch/x86/include/asm/ftrace.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index 09ad88572746..c30b3fac20ae 100644
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h
> @@ -31,6 +31,16 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
>  	return addr;
>  }
>  
> +#define ARCH_HAS_SYSCALL_MATCH_SYM_NAME
> +static inline bool arch_syscall_match_sym_name(const char *sym, const char *name)
> +{
> +	/*
> +	 * Compare the symbol name with the system call name. Skip the
> +	 * "__x64_sys" prefix.
> +	 */
> +	return !strcmp(sym + 9, name + 3);
> +}

What are the first three chars you skip for *name?

Thanks,
	Dominik