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Re: Synthetic symbol leak in in elf_x86_64_get_synthetic_symtab and elf_read_minimal_symbols


On Fri, Aug 11, 2017 at 2:27 AM, Yao Qi <qiyaoltc@gmail.com> wrote:
> On 17-08-07 10:19:15, Alex Lindsay wrote:
>>
>> We perform a couple of  dynamic allocations in
>> elf_x86_64_get_synthetic_symtab (elf64-x86-64.c):
>>
>>   s = *ret = (asymbol *) bfd_zmalloc (size);
>>
>>   names = (char *) bfd_malloc (size);
>>
>> that appear to never get freed. My patch for this:
>
> Good catch!  It is more complicated that other bfd targets allocate
> memory for asymbol in a different way as if asymbol.name is defined
> as an zero-length array, so memory allocated for both asymbol and .name
> in one bfd_malloc call, like,
>
>           sym = *r->sym_ptr_ptr;
>           if (!sym_exists_at (syms, opdsymend, symcount,
>                               sym->section->id, sym->value + r->addend))
>             {
>               ++count;
>               size += sizeof (asymbol);
>               size += strlen (syms[i]->name) + 2;
>             }
>         }
>
>       if (size == 0)
>         goto done;
>       s = *ret = bfd_malloc (size);
>
> or
>
>   size = count * sizeof (asymbol);
>   p = relplt->relocation;
>   for (i = 0; i < count; i++, p += elf32_arm_size_info.int_rels_per_ext_rel)
>     {
>       size += strlen ((*p->sym_ptr_ptr)->name) + sizeof ("@plt");
>       if (p->addend != 0)
>         size += sizeof ("+0x") - 1 + 8;
>     }
>
>   s = *ret = (asymbol *) bfd_malloc (size);
>
>>
>> diff --git a/gdb/elfread.c b/gdb/elfread.c
>> index ece704ca7c..5ed8a6f957 100644
>> --- a/gdb/elfread.c
>> +++ b/gdb/elfread.c
>> @@ -1144,6 +1144,9 @@ elf_read_minimal_symbols (struct objfile *objfile, int
>> symfile_flags,
>>
>>    if (symtab_create_debug)
>>      fprintf_unfiltered (gdb_stdlog, "Done reading minimal symbols.\n");
>> +  if (synthcount > 0)
>> +    xfree ((char *) synthsyms->name);
>
> We can't do this for some bfd targets.
>
>> +  xfree (synthsyms);
>
> We can only safely do this, but .name is leaked for x86_64.  Other
> tools using bfd, like objdump, nm, and gprof may have this issue too.
> I'll ask binutils people on asymbol allocation and de-allocation.
>

This is:

https://sourceware.org/bugzilla/show_bug.cgi?id=21943

i386 and x86-64 get_synthetic_symtab don't know if @plt should
be added to symbol name for a PLT entry.  The first pass checks
if @plt is needed and extra space is allocated in the second pass.
We can assume @plt is needed and waste some space if it isn't.

-- 
H.J.


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