Re: dladdr()

[musl <b.brezillon.musl@gmail.com>]

On 16/01/2013 17:49, Rich Felker wrote:
> On Wed, Jan 16, 2013 at 03:24:24PM +0100, musl wrote:
>>> i ran hello in gdb and it seems
>>>  sym->st_shndx==0
>>> for the "puts" symbol, i dont know the semantics of
>>> st_shndx, but the following patch makes hello.c work:
>>>
>>> diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
>>> index 935367e..ba0bd8f 100644
>>> --- a/src/ldso/dynlink.c
>>> +++ b/src/ldso/dynlink.c
>>> @@ -1178,7 +1178,7 @@ int __dladdr(void *addr, Dl_info *info)
>>>         }
>>>
>>>         for (; nsym; nsym--, sym++) {
>>> -               if (sym->st_shndx && sym->st_value
>>> +               if (/*sym->st_shndx &&*/ sym->st_value
>>>                  && (1<<(sym->st_info&0xf) & OK_TYPES)
>>>                  && (1<<(sym->st_info>>4) & OK_BINDS)) {
>>>                         void *symaddr = p->base + sym->st_value;
>>>
>>
>> You're right, sym->st_shndx only tells if the symbol is external
>> (resolved during relocation process) or internal (defined in the
>> current shared object).
> 
> st_shndx==0 is used for PLT entries. Oddly, I get (for hello.c):
> 
> Symbol table '.dynsym' contains 9 entries:
>    Num:    Value  Size Type    Bind   Vis      Ndx Name
>      0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
>      1: 00000000     0 FUNC    GLOBAL DEFAULT  UND printf
>      2: 080482c0     0 FUNC    GLOBAL DEFAULT  UND puts
>      3: 00000000     0 FUNC    GLOBAL DEFAULT  UND dladdr
> 
> So the symbol value (the address of the PLT entry) seems to be filled
> in only for symbols which are also used to take the address of a
> function. Other symbols in the PLT (which are used only for resolving
> the PLT entry at load time) lack addresses. I suspect this could
> prevent correct dladdr behavior in some cases, but I can't think of a
> serious usage case that would be broken right off.
> 
> Anyway, I'm applying nsz's patch (without the commented code :) which
> should make the situation much better (hopefully as good as glibc's).
> 
>> BTW dli_fbase is still wrong. It should be
>>
>> 	info->dli_fbase = p->map;
>>
>> and not
>>
>> 	info->dli_fbase = p->base;
> 
> I notice it disagrees with glibc, but I'm not sure I agree it's wrong.
> The man page states that dli_fbase is the "Address at which shared
> object is loaded", which is never clearly defined. On the other hand,
> dl_iterate_phdr's dlpi_addr is specified to contain the "Base address
> of object", which is defined below as "the difference between the
> virtual memory address of the shared object and the offset of that
> object in the file from which it was loaded". To me, this seems like
> the main/only "load address" that would be of interest to a program.
> However, there's also an interest in matching historical practice,
> especially since dladdr is not a standard function and the existing
> implementations could be seen as the "real" specification. Do you know
> what other systems like BSD do?


freebsd dladdr man pages defines the dli_fbase field as :
  "The base address at which the shared object is
   mapped into the address space of the calling
   process"

see:
  http://www.unix.com/man-page/FreeBSD/3/dladdr/



> 
> Rich
>