Re: [PATCH] elf: Remove fallback to the start of DT_STRTAB for dladdr

[Fangrui Song <maskray@google.com>]

On 2022-05-02, Florian Weimer wrote:
>* Fangrui Song via Libc-alpha:
>
>> When neither DT_HASH nor DT_GNU_HASH is present, the code scans
>> [DT_SYMTAB, DT_STRTAB). However, there is no guarantee that .dynstr
>> immediately follows .dynsym (e.g. lld typically places .gnu.version
>> after .dynsym).
>
>The code is compatible with lld because it always generates a hash
>table.  Maybe it was added to support old binaries without a hash table.
>So we would have to check if such binaries exist from the early
>libc.so.6 days (or check if binutils every generated ELF binaries
>without a hash table).  The glibc comment isn't clear if this was added
>because it was required at the time, or just because it seemed a good
>idea.
>
>I couldn't find any binaries with DT_SYMTAB, but without DT_HASH or
>DT_GNU_HASH in my collection, but doesn't mean that they don't exist.
>
>Thanks,
>Florian

With a linker script .hash and .gnu.hash can be removed.
But such an object behaves as if it has no symbol: symbol search will fail.
It makes sense for dladdr to not return a symbol for it.

% bmake
cc -pipe -g   -fuse-ld=bfd -g -fpic -shared -Wl,--version-script=b.ver b.c -o b.so
cc -pipe -g   -fuse-ld=bfd -g a.c -Wl,--no-as-needed -fno-pie -no-pie -Wl,-rpath=/tmp/d b.so -ldl -o a
% ./a
42

% cat b.lds   # GNU ld doesn't have this yet: https://sourceware.org/bugzilla/show_bug.cgi?id=26404
OVERWRITE_SECTIONS {
   /DISCARD/ : { *(.hash) *(.gnu.hash) }
}
% clang -fpic -fuse-ld=lld -shared b.c -Wl,b.lds -o b.so
% ./a
./a: symbol lookup error: ./a: undefined symbol: var

---

GNU ld doesn't seem to allow discarding both .gnu.hash and .hash:
  /DISCARD/ : { *(.hash) *(.gnu.hash) *(.note.GNU-stack) *(.gnu_debuglink) *(.gnu.lto_*) }

/usr/bin/ld.bfd: could not find section .hash