Re: [PATCH v8.1 14/27] xsplice, symbols: Implement symbol name resolution on address.

From: Ross Lagerwall <ross.lagerwall@citrix.com>
To: Konrad Rzeszutek Wilk <konrad@kernel.org>,
	Jan Beulich <JBeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Martin Pohlack <mpohlack@amazon.com>,
	Xen-devel <xen-devel@lists.xenproject.org>,
	sasha.levin@oracle.com
Subject: Re: [PATCH v8.1 14/27] xsplice, symbols: Implement symbol name resolution on address.
Date: Mon, 25 Apr 2016 07:41:47 +0100	[thread overview]
Message-ID: <571DBC2B.9090507@citrix.com> (raw)
In-Reply-To: <CACJDEmqkjZQSwn5yuwT3sZGztzb-5E_46P+XVxnmgxRkf+_bkA@mail.gmail.com>

On 04/22/2016 10:10 PM, Konrad Rzeszutek Wilk wrote:
>> As per my earlier reply to Konrad, there must be more to this. I.e.
>> "normal" local symbols won't get dropped together with relocations
>> referencing them getting resolved.
>
> Correct. These .LCx symbols only cover .rodata.* sections. Any other
> local symbols:
>
> [konrad@x230 x86]$ readelf --symbols prelink.o |grep bad_hypercall
>   8946: 0000000000145549     0 NOTYPE  LOCAL  DEFAULT    1 compat_bad_hypercall
>   8967: 0000000000145c79     0 NOTYPE  LOCAL  DEFAULT    1 bad_hypercall
>
> don't get dropped and do show up in the binary (nm --defined finds them).
>
> While .LC matches the type:
>   9147: 00000000000008c0     0 NOTYPE  LOCAL  DEFAULT   33 .LC5
>
> They are in four other sections:
> konrad@x230 x86]$ readelf --symbols prelink.o |grep LC | awk '{print
> $7}' | sort | uniq
> 22
> 23
> 33
> 34
>
> [22] .rodata.str1.1    PROGBITS         0000000000000000  0019d500
>        000000000000a088  0000000000000001 AMS       0     0     1
>   [23] .rodata.str1.8    PROGBITS         0000000000000000  001a7588
>        0000000000020a31  0000000000000001 AMS       0     0     8
>   [33] .init.rodata.str1 PROGBITS         0000000000000000  001d5a78
>        0000000000000a1e  0000000000000001 AMS       0     0     1
>   [34] .init.rodata.str1 PROGBITS         0000000000000000  001d6498
>        0000000000002331  0000000000000001 AMS       0     0     8
>

With some helpful investigation by Konrad, I've found this snippet in 
binutils:

   /* See if we are discarding symbols with this name.  */
   if ((flinfo->info->strip == strip_some
        && (bfd_hash_lookup (flinfo->info->keep_hash, name, FALSE, FALSE)
            == NULL))
       || (((flinfo->info->discard == discard_sec_merge
	&& (isec->flags & SEC_MERGE)
	&& !bfd_link_relocatable (flinfo->info))
        || flinfo->info->discard == discard_l)
        && bfd_is_local_label_name (input_bfd, name)))
     continue;

The default value for info->discard is discard_sec_merge, so:
Local labels referring to a mergeable section are discarded when the 
output is not relocatable.
A local label is defined as:
1) Starts with .X (for i386 unixware compilers)
2) Starts with .L
3) Starts with .. (for SRV4 compilers)
4) Starts with _.L_ (for older buggy GCC)
5) Matches L0^A.*
6) Matches L[0-9]+{^A|^B}[0-9]* (for assembler generated local labels)

So to match what is used by the default hypervisor build, I think we 
should change the check to discard when the symbol matches (2), (5), or 
(6) above and refers to a mergeable section. The above rules are defined 
in _bfd_elf_is_local_label_name and elf_i386_is_local_label_name.

-- 
Ross Lagerwall

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