Re: [PATCH] perf symbols: Fix symbol size calculation condition

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Michael Petlan <mpetlan@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	jolsa@kernel.org, atrajeev@linux.vnet.ibm.com,
	maddy@linux.ibm.com, kjain@linux.ibm.com
Subject: Re: [PATCH] perf symbols: Fix symbol size calculation condition
Date: Thu, 17 Mar 2022 12:37:00 -0300	[thread overview]
Message-ID: <YjNVnA+tToooxUV+@kernel.org> (raw)
In-Reply-To: <20220317135536.805-1-mpetlan@redhat.com>

Em Thu, Mar 17, 2022 at 02:55:36PM +0100, Michael Petlan escreveu:
> Before this patch, the symbol end address fixup to be called, needed two
> conditions being met:
> 
>   if (prev->end == prev->start && prev->end != curr->start)
> 
> Where
>   "prev->end == prev->start" means that prev is zero-long
>                              (and thus needs a fixup)
> and
>   "prev->end != curr->start" means that fixup hasn't been applied yet
> 
> However, this logic is incorrect in the following situation:
> 
> *curr  = {rb_node = {__rb_parent_color = 278218928,
>   rb_right = 0x0, rb_left = 0x0},
>   start = 0xc000000000062354,
>   end = 0xc000000000062354, namelen = 40, type = 2 '\002',
>   binding = 0 '\000', idle = 0 '\000', ignore = 0 '\000',
>   inlined = 0 '\000', arch_sym = 0 '\000', annotate2 = false,
>   name = 0x1159739e "kprobe_optinsn_page\t[__builtin__kprobes]"}
> 
> *prev = {rb_node = {__rb_parent_color = 278219041,
>   rb_right = 0x109548b0, rb_left = 0x109547c0},
>   start = 0xc000000000062354,
>   end = 0xc000000000062354, namelen = 12, type = 2 '\002',
>   binding = 1 '\001', idle = 0 '\000', ignore = 0 '\000',
>   inlined = 0 '\000', arch_sym = 0 '\000', annotate2 = false,
>   name = 0x1095486e "optinsn_slot"}
> 
> In this case, prev->start == prev->end == curr->start == curr->end,
> thus the condition above thinks that "we need a fixup due to zero
> length of prev symbol, but it has been probably done, since the
> prev->end == curr->start", which is wrong.
> 
> After the patch, the execution path proceeds to arch__symbols__fixup_end
> function which fixes up the size of prev symbol by adding page_size to
> its end offset.

You missed this:

Fixes: 3b01a413c196c910 ("perf symbols: Improve kallsyms symbol end addr calculation")

I'll add it to the patch, ok?

- Arnaldo

> Signed-off-by: Michael Petlan <mpetlan@redhat.com>
> ---
>  tools/perf/util/symbol.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index fb075aa8f8f2..dea0fc495185 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -231,7 +231,7 @@ void symbols__fixup_end(struct rb_root_cached *symbols)
>  		prev = curr;
>  		curr = rb_entry(nd, struct symbol, rb_node);
>  
> -		if (prev->end == prev->start && prev->end != curr->start)
> +		if (prev->end == prev->start || prev->end != curr->start)
>  			arch__symbols__fixup_end(prev, curr);
>  	}
>  
> -- 
> 2.18.4

-- 

- Arnaldo