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util: fix undefined behavior in wl_array_for_each

David Benjamin requested to merge davidben/wayland:wl-array-ub into main

If a wl_array has size zero, wl_array_for_each computes NULL + 0 to get to the end pointer. This should be fine, and indeed it would be fine in C++. But the C specification has a mistake here and it is actually undefined behavior. See https://davidben.net/2024/01/15/empty-slices.html

Clang's -fsanitize=undefined flags this. I ran into this in Chromium's build with wayland-scanner on one of our XML files.

../../third_party/wayland/src/src/scanner.c:1853:2: runtime error: applying zero offset to null pointer
    #0 0x55c979b8e02c in emit_code third_party/wayland/src/src/scanner.c:1853:2
    #1 0x55c979b89323 in main third_party/wayland/src/src/scanner.c
    #2 0x7f8dfdb8c6c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #3 0x7f8dfdb8c784 in __libc_start_main csu/../csu/libc-start.c:360:3
    #4 0x55c979b70f39 in _start (...)

An empty (well, minimal) XML file is sufficient to hit this case, so I've added it as a test. To reproduce, undo the fix and include only the test, then build with:

CC=clang CFLAGS="-fno-sanitize-recover=undefined" meson build/ -Db_sanitize=undefined -Db_lundef=false
ninja -C build test

Signed-off-by: David Benjamin davidben@google.com

Edited by David Benjamin

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