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_rtld_global_ro and static dlopen (was: Re: Pending fixes for all-ABIs builds)
On 11/10/2016 10:27 AM, Andreas Schwab wrote:
On Nov 10 2016, Florian Weimer <fweimer@redhat.com> wrote:
Could we put this information into rtld_global_ro, instead of having a
separate variable? (Assuming that rtld_global_ro is indeed in read-only
memory after relocation.)
It cannot be read-only, it is set only after ld.so is initialized.
I know that glibc has trouble with multiple definitions, but at least
one copy of _rtld_global_ro *is* read-only after relocation:
(gdb) print memset (&_rtld_global_ro, 1, 1)
Program received signal SIGSEGV, Segmentation fault.
__memset_sse2 () at ../sysdeps/x86_64/memset.S:110
110 movb %cl, (%rdi)
It also contains a function pointer to _dl_lookup_symbol_x, so I think
this data structure should work for locating ld.so functions.
Is the problem that we determine too late that we are running in a
nested configuration, and the inner ld.so has already been relocated?
Maybe gdb is lying to me, but it seems that we run the inner libc.so
with a _rtld_global_ro which has not been initialized beyond what is in
the default initializer. It seems that dl_main never runs.
But this means that most uses of GLRO inside libc.so.6 are broken. And
indeed, GLRO (dl_auxv) does not work as intended, and results in a NULL
pointer dereference in getauxval:
https://sourceware.org/bugzilla/show_bug.cgi?id=20802
How can we fix this? Could we synthesize a link map which interposes
the necessary definitions?
Thanks,
Florian