Michael Niedermayer:
> On Fri, Mar 18, 2022 at 11:52:54AM +0100, Andreas Rheinhardt wrote:
>> They are only needed for the fuzzer, so check for CONFIG_OSSFUZZ.
>> This decreases sizeof(FFCodec), which is important given that
>> FFCodecs reside in .data.rel.ro in case of ELF with
>> position-independent code which is always loaded and can't be shared
>> between processes.
>>
>
> They are currently only used by the fuzzer, if there is no other
> use, iam not sure. But i agree shareable memory would be better for them
>
GCC by default aligns big enough objects to 32* (so while sizeof(codec)
need not be a multiple of said alignment, the actual object is still
padded to said alignment) and it was aligned to 32 before the addition
of ch_layouts (for x64), so adding it increased the size by 32 (and
resulted in over 21KiB size increase here on a really
not-all-encompassing build). Optimizing codec_tags away would reverse this.
>
> [...]
>> diff --git a/tools/target_dec_fuzzer.c b/tools/target_dec_fuzzer.c
>> index 288aa63313..77f4bb8dd8 100644
>> --- a/tools/target_dec_fuzzer.c
>> +++ b/tools/target_dec_fuzzer.c
>> @@ -279,12 +279,14 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t
>> size) {
>> ctx->sample_rate =
>> bytestream2_get_le32(&gbc) & 0x7FFFFFFF;
>> ctx->ch_layout.nb_channels =
>> (unsigned)bytestream2_get_le32(&gbc) % FF_SANE_NB_CHANNELS;
>> ctx->block_align =
>> bytestream2_get_le32(&gbc) & 0x7FFFFFFF;
>> +#if CONFIG_OSSFUZZ
>> ctx->codec_tag =
>> bytestream2_get_le32(&gbc);
>> if (c->codec_tags) {
>> int n;
>> for (n = 0; c->codec_tags[n] != FF_CODEC_TAGS_END; n++);
>> ctx->codec_tag = c->codec_tags[ctx->codec_tag % n];
>> }
>> +#endif
>> keyframes =
>> bytestream2_get_le64(&gbc);
>> request_channel_layout =
>> bytestream2_get_le64(&gbc);
>>
>
> how does the fuzzer work without the fuzzer ?
>
You could just not use --enable-ossfuzz, but nevertheless want to "make
tools/target_dec_foo_fuzzer" (with custom fuzzer CFLAGS/LDFLAGS). But it
is not really the intended usecase, so I made sure it compiles, but
nothing more.
- Andreas
*: The ELF-x64-ABI only requires 16, but it uses 32 for compatibility
with ancient versions of GCC which required 32. One can use
-malign-data=abi to make it only use 16.
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