[PATCH] nptl: Export libthread_db-used symbols under GLIBC_PRIVATE
Carlos O'Donell
carlos@redhat.com
Mon Jun 28 02:10:29 GMT 2021
On 6/17/21 3:23 PM, Florian Weimer via Libc-alpha wrote:
> Kevin, this *almost* gives us the perfect debugging experience with your
> patched GDB in Fedora 35, according to my preliminary tests. A build
> with the patch is running here:
>
> <https://koji.fedoraproject.org/koji/buildinfo?buildID=1773177>
Please send v2 with comment update and some discussion about the missing
3 symbols that could be added (or not added).
> However, it seems that GDB still needs pthread_create in the .symtab to
> trigger loading of libpthread_db. A fully stripped libc.so.6 without
> .symtab does not trigger loading in my experiments. (The other symbols
> we preserve for valgrind's sake and are immaterial to GDB.) In other
> words,
>
> cp /lib64/libc.so.6 . ; strip libc.so.6 ; LD_LIBRARY_PATH=. gdb …
>
> does not result in working thread debugging.
This has to do with lookup_minimal_symbol() API in gdb as I noted to Simon.
I don't think gdb has loaded .dynsyms yet, but it could have.
I think we might ask Kevin or Simon to verify what is going on there.
> If the pthread_create .symtab requirement is too difficult to work
> around, we might as well put the _thread_db_* symbols into .symtab and
> tell distributions to use:
>
> strip -w -K pthread_create -K _thread_db_\* libc.so.6
>
> But then we'd need two categories of symbols in the internal consistency
> check for nptl_db (the db-symbol.awk file), so maybe that's overdoing
> things for a slightly smaller .dynsym size.
>
> Thanks,
> Florian
>
> 8<------------------------------------------------------------------8<
Note: Patch starts here.
> This allows distributions to strip debugging information from
> libc.so.6 without impacting the debugging experience.
>
> nptl_version had to be renamed to __nptl_version to avoid
> namespace issues.
> ---
> nptl/Versions | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++
> nptl/pthread_create.c | 13 +++++++-----
> nptl_db/Makefile | 2 +-
> nptl_db/db-symbols.awk | 4 +++-
> nptl_db/structs.def | 2 +-
> nptl_db/td_ta_new.c | 2 +-
> 6 files changed, 70 insertions(+), 9 deletions(-)
>
> diff --git a/nptl/Versions b/nptl/Versions
> index 62bb939d54..c03ed92848 100644
> --- a/nptl/Versions
> +++ b/nptl/Versions
> @@ -403,10 +403,14 @@ libc {
> __nptl_deallocate_tsd;
> __nptl_death_event;
> __nptl_free_tcb;
> + __nptl_last_event;
OK. Defined with DB_VARIABLE.
> __nptl_nthreads;
> + __nptl_rtld_global;
OK. Defined with DB_VARIABLE.
> __nptl_setxid_sighandler;
> __nptl_stack_list_add;
> __nptl_stack_list_del;
> + __nptl_threads_events;
> + __nptl_version;
OK. Both defined with DB_SYMBOL.
> __pthread_attr_copy;
> __pthread_attr_destroy;
> __pthread_attr_init;
> @@ -430,6 +434,58 @@ libc {
> __pthread_unwind;
> __sched_fifo_max_prio;
> __sched_fifo_min_prio;
I audited this against the current pristine upstream.
I see 3 additional symbols not in your list:
_thread_db___nptl_initial_report_events (DB_RTLD_VARIABLE)
_thread_db___nptl_nthreads (DB_MAIN_VARIABLE)
_thread_db___pthread_keys (DB_MAIN_ARRAY_VARIABLE)
Given that they *are* party of the db, is there any reason
not to include them for completeness?
So something like this...
diff --git a/nptl/Versions b/nptl/Versions
index 0a366a8a29..bd0b56dc7c 100644
--- a/nptl/Versions
+++ b/nptl/Versions
@@ -434,6 +434,8 @@ libc {
__pthread_unwind;
__sched_fifo_max_prio;
__sched_fifo_min_prio;
+ _thread_db___nptl_nthreads;
+ _thread_db___pthread_keys;
_thread_db___nptl_last_event;
_thread_db___nptl_rtld_global;
_thread_db_const_thread_area;
@@ -572,5 +574,6 @@ ld {
GLIBC_PRIVATE {
__nptl_initial_report_events;
__nptl_set_robust_list_avail;
+ _thread_db___nptl_initial_report_events;
}
}
diff --git a/nptl_db/db-symbols.awk b/nptl_db/db-symbols.awk
index 2033f95e38..257a213d5c 100644
--- a/nptl_db/db-symbols.awk
+++ b/nptl_db/db-symbols.awk
@@ -3,9 +3,7 @@
BEGIN {
%define DB_RTLD_VARIABLE(name) /* Nothing. */
-%define DB_MAIN_VARIABLE(name) /* Nothing. */
-%define DB_MAIN_SYMBOL(name) /* Nothing. */
-%define DB_MAIN_ARRAY_VARIABLE(name) /* Nothing. */
+%define DB_RTLD_GLOBAL_FIELD(name) /* Nothing. */
%define DB_LOOKUP_NAME(idx, name) required[STRINGIFY (name)] = 1;
%define DB_LOOKUP_NAME_TH_UNIQUE(idx, name) th_unique[STRINGIFY (name)] = 1;
%include "db-symbols.h"
---
Adds _thread_db___nptl_nthreads, and _thread_db___pthread_keys, which
are there already, and because we drop DB_MAIN_VARIABLE and DB_MAIN_SYMBOL
from the db-symbolc.awk exclusion list we have testing which covers *both*
verifying the _thread_db* symbol is there and the underlying variable or
symbol is there.
e.g.
cat nptl_db/db-symbols.out | grep __pthread_keys
_thread_db___pthread_keys@@GLIBC_PRIVATE ok
__pthread_keys@@GLIBC_PRIVATE ok
cat nptl_db/db-symbols.out | grep __nptl_nthreads
__nptl_nthreads@@GLIBC_PRIVATE ok
_thread_db___nptl_nthreads@@GLIBC_PRIVATE ok
So we increased coverage.
But we had remove the DB_RTLD_GLOBAL_FIELD entries because they are just
offsets into __nptl_rtld_global and we can't test for those easily.
> + _thread_db___nptl_last_event;
> + _thread_db___nptl_rtld_global;
> + _thread_db_const_thread_area;
> + _thread_db_dtv_dtv;
> + _thread_db_dtv_slotinfo_gen;
> + _thread_db_dtv_slotinfo_list_len;
> + _thread_db_dtv_slotinfo_list_next;
> + _thread_db_dtv_slotinfo_list_slotinfo;
> + _thread_db_dtv_slotinfo_map;
> + _thread_db_dtv_t_counter;
> + _thread_db_dtv_t_pointer_val;
> + _thread_db_link_map_l_tls_modid;
> + _thread_db_link_map_l_tls_offset;
> + _thread_db_list_t_next;
> + _thread_db_list_t_prev;
> + _thread_db_pthread_cancelhandling;
> + _thread_db_pthread_dtvp;
> + _thread_db_pthread_eventbuf;
> + _thread_db_pthread_eventbuf_eventmask;
> + _thread_db_pthread_eventbuf_eventmask_event_bits;
> + _thread_db_pthread_key_data_data;
> + _thread_db_pthread_key_data_level2_data;
> + _thread_db_pthread_key_data_seq;
> + _thread_db_pthread_key_struct_destr;
> + _thread_db_pthread_key_struct_seq;
> + _thread_db_pthread_list;
> + _thread_db_pthread_nextevent;
> + _thread_db_pthread_report_events;
> + _thread_db_pthread_schedparam_sched_priority;
> + _thread_db_pthread_schedpolicy;
> + _thread_db_pthread_specific;
> + _thread_db_pthread_start_routine;
> + _thread_db_pthread_tid;
> + _thread_db_register32;
> + _thread_db_register32_thread_area;
> + _thread_db_register64;
> + _thread_db_register64_thread_area;
These four are defined in db-symbols.h.
> + _thread_db_rtld_global__dl_stack_used;
> + _thread_db_rtld_global__dl_stack_user;
> + _thread_db_rtld_global__dl_tls_dtv_slotinfo_list;
> + _thread_db_sizeof_dtv_slotinfo;
> + _thread_db_sizeof_dtv_slotinfo_list;
> + _thread_db_sizeof_list_t;
> + _thread_db_sizeof_pthread;
> + _thread_db_sizeof_pthread_key_data;
> + _thread_db_sizeof_pthread_key_data_level2;
> + _thread_db_sizeof_pthread_key_struct;
> + _thread_db_sizeof_td_eventbuf_t;
> + _thread_db_sizeof_td_thr_events_t;
> + _thread_db_td_eventbuf_t_eventdata;
> + _thread_db_td_eventbuf_t_eventnum;
> + _thread_db_td_thr_events_t_event_bits;
> }
> }
>
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index 3f017f1e26..d1b6817a81 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -43,21 +43,24 @@
>
>
> /* Globally enabled events. */
> -static td_thr_events_t __nptl_threads_events __attribute_used__;
> +td_thr_events_t __nptl_threads_events __attribute__ ((nocommon));
> +libc_hidden_proto (__nptl_threads_events)
> +libc_hidden_data_def (__nptl_threads_events)
OK.
>
> /* Pointer to descriptor with the last event. */
> -static struct pthread *__nptl_last_event __attribute_used__;
> +struct pthread *__nptl_last_event __attribute__ ((nocommon));
> +libc_hidden_proto (__nptl_last_event)
> +libc_hidden_data_def (__nptl_last_event)
OK.
>
> #ifdef SHARED
> /* This variable is used to access _rtld_global from libthread_db. If
> GDB loads libpthread before ld.so, it is not possible to resolve
> _rtld_global directly during libpthread initialization. */
> -static struct rtld_global *__nptl_rtld_global __attribute_used__
> - = &_rtld_global;
> +struct rtld_global *__nptl_rtld_global = &_rtld_global;
OK.
> #endif
>
> /* Version of the library, used in libthread_db to detect mismatches. */
> -static const char nptl_version[] __attribute_used__ = VERSION;
> +const char __nptl_version[] = VERSION;
OK.
>
> /* This performs the initialization necessary when going from
> single-threaded to multi-threaded mode for the first time. */
> diff --git a/nptl_db/Makefile b/nptl_db/Makefile
> index ea721c1dcf..4cc51c0e7b 100644
> --- a/nptl_db/Makefile
> +++ b/nptl_db/Makefile
> @@ -57,7 +57,7 @@ include ../Rules
>
> $(objpfx)db-symbols.out: $(objpfx)db-symbols.v.i \
> $(common-objpfx)libc.so
> - LC_ALL=C $(READELF) -W -s $(filter %.so,$^) | $(AWK) -f $< > $@; \
> + LC_ALL=C $(READELF) -W -D -s $(filter %.so,$^) | $(AWK) -f $< > $@; \
OK. Use .dynsyms.
> $(evaluate-test)
>
> $(objpfx)db-symbols.v.i: db-symbols.awk
> diff --git a/nptl_db/db-symbols.awk b/nptl_db/db-symbols.awk
> index 6f326cf379..2033f95e38 100644
> --- a/nptl_db/db-symbols.awk
> +++ b/nptl_db/db-symbols.awk
Comment at the start of db-symbols.awk needs fixing.
Suggest:
# This script processes the output of 'readelf -W -D -s' on the libc.so
# we've just built. It checks for all the symbols used in td_symbol_list.
> @@ -13,7 +13,7 @@ BEGIN {
> in_symtab = 0;
> }
>
> -/Symbol table '.symtab'/ { in_symtab=1; next }
> +/Symbol table for image/ { in_symtab=1; next }
OK. Changed due to -D usage.
> NF == 0 { in_symtab=0; next }
>
> !in_symtab { next }
> @@ -24,6 +24,7 @@ END {
> status = 0;
>
> for (s in required) {
> + s = s "@@GLIBC_PRIVATE"
OK. Agreed, should be private.
> if (s in seen) print s, "ok";
> else {
> status = 1;
> @@ -33,6 +34,7 @@ END {
>
> any = "";
> for (s in th_unique) {
> + s = s "@@GLIBC_PRIVATE"
OK. Agreed, should be private.
> if (s in seen) {
> any = s;
> break;
> diff --git a/nptl_db/structs.def b/nptl_db/structs.def
> index 6a726f207e..e2e51acc39 100644
> --- a/nptl_db/structs.def
> +++ b/nptl_db/structs.def
> @@ -77,7 +77,7 @@ DB_STRUCT (td_eventbuf_t)
> DB_STRUCT_FIELD (td_eventbuf_t, eventnum)
> DB_STRUCT_FIELD (td_eventbuf_t, eventdata)
>
> -DB_SYMBOL (nptl_version)
> +DB_SYMBOL (__nptl_version)
OK. Namespace clean using leading __.
> DB_MAIN_SYMBOL (__nptl_create_event)
> DB_MAIN_SYMBOL (__nptl_death_event)
> DB_SYMBOL (__nptl_threads_events)
> diff --git a/nptl_db/td_ta_new.c b/nptl_db/td_ta_new.c
> index 501d922ea2..eeca29d5a0 100644
> --- a/nptl_db/td_ta_new.c
> +++ b/nptl_db/td_ta_new.c
> @@ -39,7 +39,7 @@ td_ta_new (struct ps_prochandle *ps, td_thragent_t **ta)
> LOG ("td_ta_new");
>
> /* Check whether the versions match. */
> - if (td_lookup (ps, SYM_nptl_version, &versaddr) != PS_OK)> + if (td_lookup (ps, SYM___nptl_version, &versaddr) != PS_OK)
OK. Internally consistent. Matches internal definition.
> return TD_NOLIBTHREAD;
> if (ps_pdread (ps, versaddr, versbuf, sizeof (versbuf)) != PS_OK)
> return TD_ERR;
>
--
Cheers,
Carlos.
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