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Should strip discard the .ctf section ?

Fangrui Song i@maskray.me
Tue Nov 5 02:04:00 GMT 2019 
On 2019-10-08, Nick Alcock wrote:
>On 8 Oct 2019, Nick Alcock verbalised:
>
>> (Hah! Thank you for spotting a horrible mistake on my part: see below.
>> Fix forthcoming.)
>
>Oh wait no, I am hallucinating and forgetting my own code.
>
>> On 8 Oct 2019, Fangrui Song said:
>>> --strip-all removes .symtab and .strtab. If .ctf references .symtab but
>>> is not stripped along with .symtab and .strtab, the resulting .ctf will
>>> just be broken.
>>
>> ... good point! This is clearly a horrible mistake on my part. Phew,
>> thank goodness you spotted this one early.
>>
>> The gain here is mostly from sharing symbol name strings, so I think we
>> should be sharing with .dynstr, since that's where the symtab strings
>> are kept and frankly it's the only symtab with significant amounts of
>> useful content in a stripped executable. Illumos uses the concatenation
>> of .dynstr and .SUNW_ldynsym, the latter being an executable-only string
>> table used to allow symtab lookups to work on stripped executables. We
>> don't have .ldynsym (not yet, anyway) so sharing with .dynstr alone is
>> clearly right.
>>
>> (It would be nice if we could share with both that *and* .strtab, but
>> since .strtab is hardly ever there I don't think this is likely to be
>> worth doing. We could share with .strtab in statically-linked
>> executables only but I don't think they're common enough to be worth
>> special-casing.)
>
>It turns out that I thought of this already, and even wrote a comment
>about it, and forgot that I'd done so. We specifically use the
>symstrtab, i.e. whatever string table the symbol names are in, and use
>the symtab's sh_link to find it at ctf_open time: so, on ELF, we end up
>doing the right thing automatically. See the call to examine_strtab() in
>elf_link_swap_symbols_out(), and the use of sh_link in
>libctf/ctf-open-bfd.c:ctf_bfdopen_ctfsect().
>
>Phew! Sorry for the mislead. We are fine here I think.
>
>I will make a special note in the spec, since it is clear that this is
>something that is easily got wrong! (In partial mitigation, I haven't
>slept for two days...)

strip/objcopy --keep-section=.ctf is available now
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=64f52b3ec1853d14293d5a461d7abc8bde5e3f62

We can make --strip-all and --strip-debug remove .ctf and teach the
package managers to use --keep-section=.ctf

In an ideal world, I would want --strip-all to do
https://github.com/llvm/llvm-project/blob/master/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp#L498 (llvm-objcopy --strip-all),
but unfortunately strip/objcopy --strip-all keeps many non-SHF_ALLOC sections
https://github.com/llvm/llvm-project/blob/master/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp#L459 (llvm-objcopy --strip-all-gnu).
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