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Re: [patch] Re: Accessing tls variables across files causes a bug
On Wed, 06 Aug 2008 13:16:50 +0200, Vinay Sridhar wrote:
> On Tue, 2008-08-05 at 08:21 -0400, Daniel Jacobowitz wrote:
> > On Tue, Aug 05, 2008 at 02:08:47PM +0530, Vinay Sridhar wrote:
> > > 2. obtain symbol info from msymtab and check for the section value.
> > > AFAIK, elf has section=17 for tls.
> >
> > This number doesn't mean anything. You want the STT_TLS section type.
> > But if you're doing the right thing without a minimal symbol why rely
> > on a minimal symbol at all?
> >
>
> We need to determine that the variable is a "tls" variable. When this is
> extern, the symbol is LOC_UNRESOLVED. So the section type from "sym" is
> not filled. Thats why I tried to determine this from the minimal symbol.
The real problem is that read_var_value() for LOC_UNRESOLVED should return the
address through target_translate_tls_address() while currently it returns only
SYMBOL_VALUE_ADDRESS(). In fact SYMBOL_VALUE_ADDRESS() should call
target_translate_tls_address() but that does not work as it is now being used
both as a getter and setter. If SYMBOL_VALUE_ADDRESS() should represent only
the raw symbol value (and not the TLS-translated real address) most other GDB
places should use the translated variant instead of SYMBOL_VALUE_ADDRESS().
Another possibility is that LOC_UNRESOLVED may no longer be needed for recent
gcc debuginfos always(?) containting `DW_AT_location's, therefore we would not
have to deal with `minimal_symbol's in this case at all.
> If there is any other method of determining a variable is "tls" before
> its owning symtab is linked in, could you please inform?
You cannot as in the debuginfo the only TLS indication is the presence of
DW_OP_GNU_push_tls_address in the DW_AT_location tag.
For minimal symbols one can query for `minimal_symbol's
ginfo.bfd_section->flags & SEC_THREAD_LOCAL
, for specific symbols one would have to store the BSF_THREAD_LOCAL flag from
BFD into the GDB structures. Unaware if STT_TLS symbols are always just in
the SHF_TLS sections.
> On Tue, 2008-08-05 at 08:21 -0400, Daniel Jacobowitz wrote:
> > On Tue, Aug 05, 2008 at 02:08:47PM +0530, Vinay Sridhar wrote:
> > > AFAIK, elf has section=17 for tls.
> >
> > This number doesn't mean anything. You want the STT_TLS section type.
To explain more the Daniel J.'s answer - the order of the sections is very
random and while in this compilation case the STT_TLS section was 17th in the
sections list in other compilations it will get a different number.
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[18] .tbss NOBITS 0000000000600754 00000754
0000000000000005 0000000000000000 WAT 0 0 4
(Flags=T)
Regards,
Jan