[libFuzzer] Fix incorrect coverage number in fork mode

[yescallop]

Closes #82307.

I built LLVM with the changes and tested fuzzing in fork mode. The coverage number was correct:

[ye@ye-arch ~]$ /home/ye/work/llvm-project/build/bin/clang++ -fsanitize=fuzzer test_fuzzer.cc
[ye@ye-arch ~]$ ./a.out corpus -fork=4
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 3152497917
INFO: Loaded 1 modules   (40 inline 8-bit counters): 40 [0x5aa6f7b310d0, 0x5aa6f7b310f8), 
INFO: Loaded 1 PC tables (40 PCs): 40 [0x5aa6f7b310f8,0x5aa6f7b31378), 
INFO: -fork=4: fuzzing in separate process(s)
INFO: -fork=4: 56 seed inputs, starting to fuzz in /tmp/libFuzzerTemp.FuzzWithFork54465.dir
#600649: cov: 36 ft: 224 corp: 56 exec/s: 300324 oom/timeout/crash: 0/0/0 time: 2s job: 1 dft_time: 0
#1548208: cov: 36 ft: 224 corp: 56 exec/s: 315853 oom/timeout/crash: 0/0/0 time: 3s job: 2 dft_time: 0
#2465991: cov: 36 ft: 224 corp: 56 exec/s: 229445 oom/timeout/crash: 0/0/0 time: 4s job: 3 dft_time: 0
#3887877: cov: 36 ft: 224 corp: 56 exec/s: 284377 oom/timeout/crash: 0/0/0 time: 5s job: 4 dft_time: 0

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[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Scallop Ye (yescallop)

Changes

Closes #82307.

I tried to build LLVM and test the changes but failed with the following error:

/usr/bin/ld: cannot find /home/ye/work/llvm-project/build/lib/clang/19/lib/linux/libclang_rt.fuzzer-x86_64.a: No such file or directory
/usr/bin/ld: cannot find /home/ye/work/llvm-project/build/lib/clang/19/lib/linux/libclang_rt.fuzzer_interceptors-x86_64.a: No such file or directory
/usr/bin/ld: cannot find /home/ye/work/llvm-project/build/lib/clang/19/lib/linux/libclang_rt.ubsan_standalone-x86_64.a: No such file or directory
/usr/bin/ld: cannot find /home/ye/work/llvm-project/build/lib/clang/19/lib/linux/libclang_rt.ubsan_standalone_cxx-x86_64.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

I have no idea how to fix this. It'd be nice if you could help me with testing the changes.

---

Full diff: https://github.com/llvm/llvm-project/pull/82335.diff

1 Files Affected:

• (modified) compiler-rt/lib/fuzzer/FuzzerFork.cpp (+1-1)

diff --git a/compiler-rt/lib/fuzzer/FuzzerFork.cpp b/compiler-rt/lib/fuzzer/FuzzerFork.cpp
index c248a1d246a306..e544cd846e4db5 100644
--- a/compiler-rt/lib/fuzzer/FuzzerFork.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerFork.cpp
@@ -349,7 +349,7 @@ void FuzzWithFork(Random &Rand, const FuzzingOptions &Options,
                         &NewFeatures, Env.Cov, &NewCov, CFPath,
                         /*Verbose=*/false, /*IsSetCoverMerge=*/false);
     Env.Features.insert(NewFeatures.begin(), NewFeatures.end());
-    Env.Cov.insert(NewFeatures.begin(), NewFeatures.end());
+    Env.Cov.insert(NewCov.begin(), NewCov.end());
     RemoveFile(CFPath);
   }
 

[yescallop]

Ping

[renatahodovan]

The fix also works for me.

[forzafedor]

Ping

[Schtolc]

Ping

[vitalybuka]

Would be possible to add a test?

[vitalybuka]

Thanks for the patch!

I assume no volunteers to add, a test, and I am not ready to do that myself.

But the code with the patch is better than without, so no reason to block it.

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