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Date:   Thu, 17 Mar 2022 18:49:35 +0100 (CET)
From:   Michael Petlan <mpetlan@redhat.com>
To:     Arnaldo Carvalho de Melo <acme@kernel.org>
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
In-Reply-To: <YjNVnA+tToooxUV+@kernel.org>
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References: <20220317135536.805-1-mpetlan@redhat.com> <YjNVnA+tToooxUV+@kernel.org>
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On Thu, 17 Mar 2022, Arnaldo Carvalho de Melo wrote:
> 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?

Sure thing. Thanks.
M

> 
> - 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
> 
>