Warning -Warray-bounds on H_ns in src/util/srp.cpp

[darnuria]

Minetest version

Commit: 3ce03d1
Type: Build issue / warning

OS / Hardware

Operating system: Ubuntu 18.04
Compiler: Gcc/G++ (Ubuntu 10.1.0-2ubuntu1~18.04) 10.1.0

Summary

Got this compilation warning on build with gcc 10.

Steps to reproduce

Compile with gcc 10.

[ 31%] Building CXX object src/CMakeFiles/minetest.dir/version.cpp.o
In file included from /usr/include/string.h:494,
                 from /usr/include/c++/10/cstring:42,
                 from minetest/src/util/srp.cpp:43:
In function ‘void* memcpy(void*, const void*, size_t)’,
    inlined from ‘int calculate_x(__mpz_struct*, SRP_HashAlgorithm, const unsigned char*, size_t, const char*, const unsigned char*, size_t)’ at minetest/src/util/srp.cpp:432:8:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:34:33: warning: ‘void* __builtin_memcpy(void*, const void*, long unsigned int)’ forming offset [64, 4294967294] is out of the bounds [0, 64] of object ‘ucp_hash’ with type ‘unsigned char [64]’ [-Warray-bounds]
   34 |   return __builtin___memcpy_chk (__dest, __src, __len, __bos0 (__dest));
      |          ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
minetest/src/util/srp.cpp: In function ‘int calculate_x(__mpz_struct*, SRP_HashAlgorithm, const unsigned char*, size_t, const char*, const unsigned char*, size_t)’:
minetest/src/util/srp.cpp:443:16: note: ‘ucp_hash’ declared here
  443 |  unsigned char ucp_hash[SHA512_DIGEST_LENGTH];
      |                ^~~~~~~~

Link to code:

https://github.com/minetest/minetest/blob/master/src/util/srp.cpp#L424-L437

Previous bug on this line

#6855

[numberZero]

Finally proper compiler warning (on my system it was reporting -1 as the out of bounds index).
The code here assumes that hash_length(alg) <= SHA512_DIGEST_LENGTH for each and every hashing algorithm that may ever reach the point. The only supported algorithm seems to be SHA256, so... that seems to be false alert, if treated literally; OTOH that should be clear from the code (like defining and using MAX_DIGEST_LENGTH or so).