Use FMT_THROW in fuzzing build mode

[asraa]

Make code properly build in fuzz mode with clang/gcc when -fno-exceptions is specified by using FMT_THROW in format files when building with FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION.

Signed-off-by: Asra Ali asraa@google.com

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Could you elaborate why this is needed? The {fmt}'s fuzz tests are always run with exceptions enabled in oss-fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/list?colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20Summary&q=proj%3Dlibfmt&can=1

[asraa]

Thanks! We have projects (Envoy, and others) that depend on fmt which are built with no exception mode, and are also fuzzed. These project's fuzz targets cannot build with the same production fmt lib dependency with exceptions disabled. Ideally, those project's fuzzers should model their production environment, they do not expect throws in their dependency.

[vitaut]

I think a better fix would be to replace FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION with an {fmt}-specific macro (e.g. FMT_FUZZING) set when the FMT_FUZZ CMake compiler option is enabled:

fmt/CMakeLists.txt

Line 309 in 9dde9f0

if (FMT_FUZZ)

[asraa]

Thank you, that would work well for us! I will update the PR and fuzzing README to reflect that.

[vitaut]

Is FUZZ_MODE documented anywhere?

[asraa]

Sorry about that -- I mixed up the macros. I tested the fuzz targets in this branch and in master branch and didn't hit any OOMs at least while running fuzzer_one_arg

[asraa]

Both master and this branch start around 100k+ exec/sec, but my branch will go down to 40k sometimes. Any idea why that's happening?

[vitaut]

Looks good but please

1. Define FMT_FUZZ in CMake:

   fmt/CMakeLists.txt

   Line 309 in f2ed03b

   if (FMT_FUZZ)

    target_compile_definitions(fmt PUBLIC FMT_FUZZ)

2. Use #ifdef instead of #if to prevent warnings when FMT_FUZZ is not defined.

Any idea why that's happening?

Not sure.

[asraa]

Done -- retested and the fuzzer now maintains 100k+ speed, I wonder if the warnings were slowing it down? Thank you!

[vitaut]

Thank you!