[PATCH 1/2] Replace __libc_multiple_libcs with tri-state __libc_type

[Florian Weimer]

* Adhemerval Zanella via Libc-alpha:

> Looks good in general, although I am not sure if '__libc_type' name
> does show its intent.  Although thing that I am not sure is if we need
> to access it through atomics (since dlmopen/static dlopen might be
> called by multiple threads and it access does not seemed to be protected
> by any lock).

The sbrk code is inherently racy because it lacks locking.

We could move it into ld.so, add locking, and thus make it safe(r) to
use from multiple namespaces (except after static dlopen).

sbrk (0) could bypass the lock in case some crash handler calls that as
part of its error reporting.

Maybe that's the way to go?

Or we could leave it in libc.so (with or without locking), and have it
fail in inner namespaces (for non-zero arguments), in which case we
could do with a two-state __libc_initial variable that has never to be
reset after initialization via __libc_early_init.  I think the
simplicity of that is quite attractive, so I'm going to try that first.

>> diff --git a/elf/libc_early_init.c b/elf/libc_early_init.c
>> index 725ab2f811..fec5d6035e 100644
>> --- a/elf/libc_early_init.c
>> +++ b/elf/libc_early_init.c
>> @@ -17,9 +17,14 @@
>>     <https://www.gnu.org/licenses/>.  */
>>  
>>  #include <ctype.h>
>> +#include <ldsodefs.h>
>>  #include <libc-early-init.h>
>> +#include <libc-internal.h>
>>  #include <sys/single_threaded.h>
>>  
>> +char __libc_type __attribute__ ((nocommon));
>> +libc_hidden_def (__libc_type)
>> +
>
> I am not sure about the __libc_type, it seems a bit too vague and does not
> indicate its real usage.  Maybe __libc_multiple_state ?

__libc_multiple if it remains tri-state?

> I also forgot why do we need a noncommon attribute here, is it to force
> a linker error on multiple definitions?

It's required because of the libc_hidden_proto/libc_hidden_def,
otherwise the alias can't be created.

Thanks,
Florian
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