From: Florian Weimer <fweimer@redhat.com>
To: Fangrui Song <maskray@google.com>
Cc: Fangrui Song via Libc-alpha <libc-alpha@sourceware.org>
Subject: Re: [PATCH] elf: Remove fallback to the start of DT_STRTAB for dladdr
Date: Mon, 02 May 2022 09:21:43 +0200 [thread overview]
Message-ID: <87k0b48kmw.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <20220502070415.kv62bjtdhhcyefjn@google.com> (Fangrui Song's message of "Mon, 2 May 2022 00:04:15 -0700")
* Fangrui Song:
> 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
This looks like it might be an lld bug. DT_HASH is mandatory in the ELF
specification. We ignore that requirement in the GNU ABI and use
DT_GNU_HASH instead, even for static PIE binaries.
Do you want to drop symbol tables from static PIE binaries?
Thanks,
Florian
next prev parent reply other threads:[~2022-05-02 7:21 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-01 21:50 Fangrui Song
2022-05-02 6:56 ` Florian Weimer
2022-05-02 7:04 ` Fangrui Song
2022-05-02 7:21 ` Florian Weimer [this message]
2022-05-02 7:30 ` Fangrui Song
2022-05-02 8:26 ` Florian Weimer
2022-05-02 8:34 ` Florian Weimer
2022-05-02 16:06 ` Fangrui Song
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