Subject: Re: [PATCH] powerpc/kprobes: Remove kretprobe_trampoline_holder.

From: Thiago Jung Bauermann <[email-masked]>
Date: Wed, 13 Apr 2016 21:43:10 -0300

Hello,

People seem to be considering patches for next, so this looks like a good 
moment to ping about this one.

-- 
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center

Am Donnerstag, 31 März 2016, 17:10:40 schrieb Thiago Jung Bauermann:
> Fixes the following testsuite failure:
> 
>   $ sudo ./perf test -v kallsyms
>    1: vmlinux symtab matches kallsyms                          :
>   --- start ---
>   test child forked, pid 12489
>   Using /proc/kcore for kernel object code
>   Looking at the vmlinux_path (8 entries long)
>   Using /boot/vmlinux for symbols
>   0xc00000000003d300: diff name v: .kretprobe_trampoline_holder k:
> kretprobe_trampoline Maps only in vmlinux:
>    c00000000086ca38-c000000000879b6c 87ca38 [kernel].text.unlikely
>    c000000000879b6c-c000000000bf0000 889b6c [kernel].meminit.text
>    c000000000bf0000-c000000000c53264 c00000 [kernel].init.text
>    c000000000c53264-d000000004250000 c63264 [kernel].exit.text
>    d000000004250000-d000000004450000 0 [libcrc32c]
>    d000000004450000-d000000004620000 0 [xfs]
>    d000000004620000-d000000004680000 0 [autofs4]
>    d000000004680000-d0000000046e0000 0 [x_tables]
>    d0000000046e0000-d000000004780000 0 [ip_tables]
>    d000000004780000-d0000000047e0000 0 [rng_core]
>    d0000000047e0000-ffffffffffffffff 0 [pseries_rng]
>   Maps in vmlinux with a different name in kallsyms:
>   Maps only in kallsyms:
>    d000000000000000-f000000000000000 1000000000010000 [kernel.kallsyms]
>    f000000000000000-ffffffffffffffff 3000000000010000 [kernel.kallsyms]
>   test child finished with -1
>   ---- end ----
>   vmlinux symtab matches kallsyms: FAILED!
> 
> The problem is that the kretprobe_trampoline symbol looks like this:
> 
>   $ eu-readelf -s /boot/vmlinux G kretprobe_trampoline
>    2431: c000000001302368     24 NOTYPE  LOCAL  DEFAULT       37
> kretprobe_trampoline_holder 2432: c00000000003d300      8 FUNC    LOCAL 
> DEFAULT        1 .kretprobe_trampoline_holder 97543: c00000000003d300    
>  0 NOTYPE  GLOBAL DEFAULT        1 kretprobe_trampoline
> 
> Its type is NOTYPE, and its size is 0, and this is a problem because
> symbol-elf.c:dso__load_sym skips function symbols that are not STT_FUNC
> or STT_GNU_IFUNC (this is determined by elf_sym__is_function). Even
> if the type is changed to STT_FUNC, when dso__load_sym calls
> symbols__fixup_duplicate, the kretprobe_trampoline symbol is dropped in
> favour of .kretprobe_trampoline_holder because the latter has non-zero
> size (as determined by choose_best_symbol).
> 
> With this patch, all vmlinux symbols match /proc/kallsyms and the
> testcase passes.
> 
> Commit c1c355ce14c0 ("x86/kprobes: Get rid of
> kretprobe_trampoline_holder()") gets rid of kretprobe_trampoline_holder
> altogether on x86. This commit does the same on powerpc. This change
> introduces no regressions on the perf and ftracetest testsuite results.
> 
> Cc: Ananth N Mavinakayanahalli <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Reviewed-by: Naveen N. Rao <[email-masked]>
> Signed-off-by: Thiago Jung Bauermann <[email-masked]>
> ---
>  arch/powerpc/kernel/kprobes.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 7c053f281406..417c0eadd094 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -278,12 +278,11 @@ no_kprobe:
>   * 	- When the probed function returns, this probe
>   * 		causes the handlers to fire
>   */
> -static void __used kretprobe_trampoline_holder(void)
> -{
> -	asm volatile(".global kretprobe_trampoline\n"
> -			"kretprobe_trampoline:\n"
> -			"nop\n");
> -}
> +asm(".global kretprobe_trampoline\n"
> +	".type kretprobe_trampoline, @function\n"
> +	"kretprobe_trampoline:\n"
> +	"nop\n"
> +	".size kretprobe_trampoline, .-kretprobe_trampoline\n");
> 
>  /*
>   * Called when the probe at kretprobe trampoline is hit