linux-kernel - Re: [PATCH 1/2] perf hists browser: Add NULL pointer check to prevent crash

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Date:	Thu, 3 Dec 2015 00:59:41 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	acme@...nel.org, lizefan@...wei.com, pi3orama@....com,
	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 1/2] perf hists browser: Add NULL pointer check to
 prevent crash

On Wed, Dec 02, 2015 at 12:51:32PM +0000, Wang Nan wrote:
> Before this patch we can trigger a segfault by following steps:
> 
>  Step 1: perf report
> 
>  Step 2: Use UP/DOWN to select an entry, don't press 'ENTER'
> 
>  Step 3: Use '/' to filter symbols, use a filter which returns
>          empty result
> 
>  Step 4: Press 'ENTER' (notice here that the old selection is still
> 		        there. This is another problem)

I guess your data file doesn't contain callchains.  Otherwise 'ENTER'
key will toggle it and not show context menu.  We now support 'm' key
to show context menu in any case.


> 
>  Step 5: Press 'ENTER' to annotate that symbol
> 
>  Step 6: Press 'LEFT' to go out.
> 
>  Result: segfault:
> 
>  perf: Segmentation fault
>  -------- backtrace --------
>  /home/wangnan/perf[0x53e568]
>  /lib64/libc.so.6(+0x3545f)[0x7fba75d3245f]
>  /home/wangnan/perf[0x537516]
>  /home/wangnan/perf[0x533fef]
>  /home/wangnan/perf[0x53b347]
>  /home/wangnan/perf(perf_evlist__tui_browse_hists+0x96)[0x53d206]
>  /home/wangnan/perf(cmd_report+0x1b9f)[0x442c7f]
>  /home/wangnan/perf[0x47efa2]
>  /home/wangnan/perf(main+0x5f5)[0x432fa5]
>  /lib64/libc.so.6(__libc_start_main+0xf4)[0x7fba75d1ebd4]
>  /home/wangnan/perf[0x4330d4]
> 
> This is because in this case 'nd' could be NULL in
> ui_browser__hists_seek(), but that function never check it.
> 
> This patch adds checker for potential NULL pointer in that function.
> After this patch the above steps won't segfault again.
> 
> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> ---
>  tools/perf/ui/browsers/hists.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/ui/browsers/hists.c b/tools/perf/ui/browsers/hists.c
> index 33da341..9458da8 100644
> --- a/tools/perf/ui/browsers/hists.c
> +++ b/tools/perf/ui/browsers/hists.c
> @@ -1280,8 +1280,10 @@ static void ui_browser__hists_seek(struct ui_browser *browser,
>  	 * Moves not relative to the first visible entry invalidates its
>  	 * row_offset:
>  	 */
> -	h = rb_entry(browser->top, struct hist_entry, rb_node);
> -	h->row_offset = 0;
> +	if (browser->top) {
> +		h = rb_entry(browser->top, struct hist_entry, rb_node);
> +		h->row_offset = 0;
> +	}

Did you have a problem with this code?  It seems that
ui_browser__hists_init_top() ensures browser->top is initialized.



>  
>  	/*
>  	 * Here we have to check if nd is expanded (+), if it is we can't go
> @@ -1297,6 +1299,8 @@ static void ui_browser__hists_seek(struct ui_browser *browser,
>  	 * and stop when we printed enough lines to fill the screen.
>  	 */
>  do_offset:
> +	if (!nd)
> +		return;

So I guess checking 'nd' being NULL here is enough.  And please add a
blank line for readability.

Thanks,
Namhyung


>  	if (offset > 0) {
>  		do {
>  			h = rb_entry(nd, struct hist_entry, rb_node);
> -- 
> 1.8.3.4
> 
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