fuzz: set the output argument of FuzzedSock::Accept() #31316

1. 
   vasild commented at 1:25 pm on November 18, 2024: contributor

   FuzzedSock::Accept() properly returns a new socket, but it forgot to set the output argument addr, like accept(2) is expected to.

   This could lead to reading uninitialized data during testing when we read it, e.g. from CService::SetSockAddr() which reads the sa_family member.

   Set addr to a fuzzed IPv4 or IPv6 address.

2. fuzz: set the output argument of FuzzedSock::Accept()

   `FuzzedSock::Accept()` properly returns a new socket, but it forgot to
   set the output argument `addr`, like `accept(2)` is expected to.
   
   This could lead to reading uninitialized data during testing when we
   read it, e.g. from `CService::SetSockAddr()` which reads the `sa_family`
   member.
   
   Set `addr` to a fuzzed IPv4 or IPv6 address.

   7e28f6d0dd
3. 
   DrahtBot commented at 1:25 pm on November 18, 2024: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31316.

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4. DrahtBot added the label Tests on Nov 18, 2024
5. 
   vasild commented at 1:26 pm on November 18, 2024: contributor

   Found in another mocked socket implementation (not FuzzedSock) in https://github.com/Sjors/bitcoin/pull/50#issuecomment-2478468037, but FuzzedSock has the same problem.
6. 
   dergoegge commented at 1:18 pm on November 20, 2024: member

   Perhaps this makes more sense in combination with a test that uses it? Currently FuzzedSock::Accept seems entirely unused: https://maflcko.github.io/b-c-cov/fuzz.coverage/src/test/fuzz/util/net.cpp.gcov.html

Contributors
vasild DrahtBot dergoegge

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