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tcbhead_t gdb access for nonthreaded, gdb for longjmp()
- From: Jan Kratochvil <jan dot kratochvil at redhat dot com>
- To: gdb at sourceware dot org
- Date: Fri, 8 Sep 2006 12:22:35 +0200
- Subject: tcbhead_t gdb access for nonthreaded, gdb for longjmp()
Hi,
crossposted to gdb+glibc as the patches closely correlate:
currently gdb/glibc cannot access TLS of debugged processes if compiled without
libpthread (nonthreaded). Command line `print errno' produces one of:
Cannot find thread-local variables on this target
Cannot access memory at address 0x8
[ gdb-20060908-tls-0.patch: gdb.threads/tls-print.exp ]
It also affects stepping over longjmp() as gdb calculates target address which
is PTR_MANGLE()d through TLS-based `pointer_guard' magic, as `next' will now:
Cannot insert breakpoint -12.
Error accessing memory address 0xb9227d3b: Input/output error.
[ gdb-20060908-tls-0.patch: gdb.threads/tls-longjmp.exp ]
I would like to get approval of this design acceptance to finish the details.
glibc part:
* Provide some access to the `tcbhead_t.pointer_guard' field for gdb.
Currently implemented by `td_thr_getxregs' providing only `pointer_guard'.
New non-Solaris `td_thr_*' function could be provided instead.
* All the `libthread_db' functions accessing inferior's `_thread_db*' symbols
of `libpthread' fallback to the new `_local_db*' symbols in `libthread_db'
itself. `libthread_db'<=>`libpthread' versions must match anyway.
I admit I do not know how may `libthread_db' and `libpthread' as there is
already required in `td_ta_new' their versions match. Anyway it should be
enough for 99% of cases - as the fallback option.
gdb part:
* `longjmp' decoder attempts to use `td_thr_getxregs', otherwise fallbacks
to get the TLS base by `ps_get_thread_area' and the offset value
`offsetof (tcbhead_t, pointer_guard)' from debuginfo, otherwise fallbacks
to internal constant offset.
* `SEC_THREAD_LOCAL' symbols are processed as a new expression data type.
* TLS variables access uses legacy `thread_db_get_thread_local_address'
as it depends on the `_local_db*' fallback implementation for nonthreaded
processes missing `libthread_db' with the legacy `_thread_db*' symbols.
`longjmp' decoder can cope without glibc support by using debuginfo instead.
I would rather like to drop this workaround and rely on the glibc support.
TLS access needs the attached glibc patch for the nonthreaded processes.
I could provide gdb decoding without glibc support but I do not like it.
It works now only on i386, x86_64 to be debugged if it is accepted this way.
Patches still contain several FIXMEs; their fixes should not change the design.
glibc part should be arch-dependent, it will now fail to compile on arches
without existing `pointer_guard'.
Behavior changes depending on:
* Application linked with libpthread or without libpthread.
* glibc original/patched by this TLS extension.
* -ggdb3 (overriding the TLS `errno' resolving just by the macro text).
* Debuginfo availability for libpthread (for `longjmp').
Unfortunately the system changes cannot be tested just by the gdb testsuite.
Thanks,
Jan
Patches at: http://www.jankratochvil.net/priv/tls/