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

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] .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)))

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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