Re: [RFC] perf tool: Fix memory corruption because of zero length symbols

[Namhyung Kim <namhyung@kernel.org>]

On Tue, Oct 24, 2017 at 07:50:06PM +0530, Ravi Bangoria wrote:
> Perf top is often crashing at very random locations on powerpc.
> After investigating, I found the crash only happens when sample
> is of zero length symbol. Powerpc kernel has many such symbols
> which does not contain length details in vmlinux binary and thus
> start and end addresses of such symbols are same.
> 
> Structure
> 
>   struct sym_hist {
>         u64                   nr_samples;
>         u64                   period;
>         struct sym_hist_entry addr[0];
>   };
> 
> has last member 'addr[]' of size zero. 'addr[]' is an array of
> addresses that belongs to one symbol (function). If function
> consist of 100 instructions, 'addr' points to an array of 100
> 'struct sym_hist_entry' elements. For zero length symbol, it
> points to the *empty* array, i.e. no members in the array and
> thus offset 0 is also invalid for such array.
> 
>   static int __symbol__inc_addr_samples(...)
>   {
>         ...
>         offset = addr - sym->start;
>         h = annotation__histogram(notes, evidx);
>         h->nr_samples++;
>         h->addr[offset].nr_samples++;
>         h->period += sample->period;
>         h->addr[offset].period += sample->period;
>         ...
>   }
> 
> Here, when 'addr' is same as 'sym->start', 'offset' becomes 0,
> which is valid for normal symbols but *invalid* for zero length
> symbols and thus updating h->addr[offset] causes memory corruption.
> 
> Fix this by adding one dummy element for zero length symbols.
> 
> Fixes: edee44be5919 ("perf annotate: Don't throw error for zero length symbols")
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>

Acked-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


> ---
>  tools/perf/util/annotate.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/annotate.c b/tools/perf/util/annotate.c
> index 4397a8b..15db525 100644
> --- a/tools/perf/util/annotate.c
> +++ b/tools/perf/util/annotate.c
> @@ -606,9 +606,19 @@ static struct arch *arch__find(const char *name)
>  int symbol__alloc_hist(struct symbol *sym)
>  {
>  	struct annotation *notes = symbol__annotation(sym);
> -	const size_t size = symbol__size(sym);
> +	size_t size = symbol__size(sym);
>  	size_t sizeof_sym_hist;
>  
> +	/*
> +	 * Add buffer of one element for zero length symbol.
> +	 * When sample is taken from first instruction of
> +	 * zero length symbol, perf still resolves it and
> +	 * shows symbol name in perf report and allows to
> +	 * annotate it.
> +	 */
> +	if (size == 0)
> +		size = 1;
> +
>  	/* Check for overflow when calculating sizeof_sym_hist */
>  	if (size > (SIZE_MAX - sizeof(struct sym_hist)) / sizeof(struct sym_hist_entry))
>  		return -1;
> -- 
> 1.8.3.1
>