Using libthread_db.so with single-threaded programs, for TLS access (was: Re: [RFC PATCH 0/3] Pretty-printing for errno)
Pedro Alves <[email protected]>
| Newsgroups | gmane.comp.lib.glibc.alpha,gmane.comp.gdb.devel |
|---|---|
| Message-ID | <[email protected]> |
On 09/07/2017 12:34 PM, Pedro Alves wrote:
> On 09/06/2017 10:03 PM, Zack Weinberg wrote:
>
>> So, changes to both gdb and libthread_db seem to be required here. I
>> do think that _in principle_ it ought to be possible to use
>> libthread_db to retrieve the address of thread-local data even if the
>> inferior is not linked with libpthread; glibc has quite a few
>> thread-specific variables (errno most prominent, of course, but also
>> h_errno, _res, etc), and so might any library which can be used from
>> both single- and multithreaded programs.
>>
>> This is really not code I feel comfortable hacking up, though, and
>> it's probably more of a project than I have time for, in any case.
>
> Sounds like a promising approach though. I'd like to see this path
> explored a bit more. I'll keep this in my TODO, even though it's
> not likely to bubble up very soon. Thanks for the discussion/ideas!
So I played with this a bit more on the plane back from Cauldron,
to try to see if we'd hit some major roadblock. I also chatted
with Carlos a bit about this back at the Cauldron, and seemingly
there's no major reason this can't be made to work,
TLS-internals-wise.
Seems like that it's mainly a case of moving libthread_db.so-related
symbols from libpthread.so elsewhere. More below.
I hacked libthread_db.so to disable the nptl_version check, so that
it always successfully loads with non-threaded programs. And then
I tweaked GDB enough to make it actually reach libthread_db.so's
td_thr_tls_get_addr in that scenario too. That's when I hit
another snag: the symbols that libthread_db.so needs which describe
the necessary offsets of internal data structures for getting at the
TLS blocks are also in libpthread.so... In particular, the first
we stumble on is "_thread_db_link_map_l_tls_modid". I made GDB
print the symbol lookups to make it easier to debug. Vis:
(gdb) p errno
ps_pglobal_lookup: name="__stack_user" => PS_NOSYM
warning: Cannot find user-level thread for LWP 31772: generic error
ps_pglobal_lookup: name="_thread_db_link_map_l_tls_modid" => PS_NOSYM
Cannot find thread-local storage for process 31772, shared library /lib64/libc.so.6:
operation not applicable to
The lookup is coming from here:
(top-gdb) bt
#0 ps_pglobal_lookup (ph=0x1f65fe0, obj=0x7fffe58f93ae "libpthread.so.0", name=0x7fffe58f9e48 "_thread_db_link_map_l_tls_modid", sym_addr=0x7fffffffc428) at src/gdb/proc-service.c:115
#1 0x00007fffe58f88a8 in td_mod_lookup (ps=<optimized out>, mod=mod@entry=0x7fffe58f93ae "libpthread.so.0", idx=<optimized out>, sym_addr=sym_addr@entry=0x7fffffffc428)
at td_symbol_list.c:48
#2 0x00007fffe58f8f45 in _td_locate_field (ta=ta@entry=0x1f84df0, desc=desc@entry=0x1f84fbc, descriptor_name=descriptor_name@entry=43, idx=idx@entry=0x0,
address=address@entry=0x7fffffffc458) at fetch-value.c:54
#3 0x00007fffe58f8ff0 in _td_fetch_value (ta=0x1f84df0, desc=0x1f84fbc, descriptor_name=descriptor_name@entry=43, idx=idx@entry=0x0, address=0x7ffff7ff7658,
result=result@entry=0x7fffffffc498) at fetch-value.c:94
#4 0x00007fffe58f8ddf in td_thr_tls_get_addr (th=0x7fffffffc4e0, map_address=<optimized out>, offset=16, address=0x7fffffffc4f8) at td_thr_tls_get_addr.c:31
...
So we'd need to move that symbol (and maybe others) to one of ld.so/libc.so
instead. AFAICT, those magic symbols are described in nptl_db/structs.def.
I haven't looked enough to figure out what ends up expanding those macros
in libpthread.so. This is where I stopped.
I'm attaching the gdb and libthread_db.so patches I used, both against current
master in their respective projects. See comments within the patches.
I've also pushed the gdb patch to a "users/palves/tls-nonthreaded" branch.
(I don't think I have write access to glibc's git.)
Thanks,
Pedro Alves
gdb.patch
(text/x-patch, 4.8 KB)
From 24f816f528fa110b2dc670a5f43899b60983ac2b Mon Sep 17 00:00:00 2001 From: Pedro Alves <[email protected]> Date: Wed, 13 Sep 2017 11:43:15 +0100 Subject: [PATCH] Use libthread_db.so with non-threaded programs, for TLS --- gdb/linux-thread-db.c | 39 ++++++++++++++++++++++++++++++--------- gdb/proc-service.c | 19 ++++++++++++++++--- 2 files changed, 46 insertions(+), 12 deletions(-) diff --git a/gdb/linux-thread-db.c b/gdb/linux-thread-db.c index 0e16f6a..c2fea86 100644 --- a/gdb/linux-thread-db.c +++ b/gdb/linux-thread-db.c @@ -104,7 +104,7 @@ set_libthread_db_search_path (char *ignored, int from_tty, /* If non-zero, print details of libthread_db processing. */ -static unsigned int libthread_db_debug; +unsigned int libthread_db_debug; static void show_libthread_db_debug (struct ui_file *file, int from_tty, @@ -354,13 +354,19 @@ thread_from_lwp (ptid_t ptid) err = info->td_ta_map_lwp2thr_p (info->thread_agent, ptid_get_lwp (ptid), &th); if (err != TD_OK) - error (_("Cannot find user-level thread for LWP %ld: %s"), - ptid_get_lwp (ptid), thread_db_err_str (err)); + { + warning (_("Cannot find user-level thread for LWP %ld: %s"), + ptid_get_lwp (ptid), thread_db_err_str (err)); + return NULL; + } err = info->td_thr_get_info_p (&th, &ti); if (err != TD_OK) - error (_("thread_get_info_callback: cannot get thread info: %s"), - thread_db_err_str (err)); + { + warning (_("thread_get_info_callback: cannot get thread info: %s"), + thread_db_err_str (err)); + return NULL; + } /* Fill the cache. */ tp = find_thread_ptid (ptid); @@ -1429,7 +1435,7 @@ thread_db_get_thread_local_address (struct target_ops *ops, if (thread_info != NULL && thread_info->priv == NULL) thread_info = thread_from_lwp (ptid); - if (thread_info != NULL && thread_info->priv != NULL) + if (true) { td_err_e err; psaddr_t address; @@ -1437,6 +1443,21 @@ thread_db_get_thread_local_address (struct target_ops *ops, info = get_thread_db_info (ptid_get_pid (ptid)); + /* Handle executables that don't link with pthread. We still + use libthread_db.so with those in order to be able to access + TLS variables. This bakes in awareness that the main + thread's special handle is "0". Should maybe make + td_ta_map_lwp2thr return that handle instead for non-threaded + programs. */ + td_thrhandle_t main_thr_th {}; + main_thr_th.th_ta_p = info->thread_agent; + + td_thrhandle_t *th; + if (thread_info == NULL || thread_info->priv == NULL) + th = &main_thr_th; + else + th = &thread_info->priv->th; + /* Finally, get the address of the variable. */ if (lm != 0) { @@ -1448,7 +1469,8 @@ thread_db_get_thread_local_address (struct target_ops *ops, /* Note the cast through uintptr_t: this interface only works if a target address fits in a psaddr_t, which is a host pointer. So a 32-bit debugger can not access 64-bit TLS through this. */ - err = info->td_thr_tls_get_addr_p (&thread_info->priv->th, + + err = info->td_thr_tls_get_addr_p (th, (psaddr_t)(uintptr_t) lm, offset, &address); } @@ -1466,8 +1488,7 @@ thread_db_get_thread_local_address (struct target_ops *ops, PR libc/16831 due to GDB PR threads/16954 LOAD_MODULE is also NULL. The constant number 1 depends on GNU __libc_setup_tls initialization of l_tls_modid to 1. */ - err = info->td_thr_tlsbase_p (&thread_info->priv->th, - 1, &address); + err = info->td_thr_tlsbase_p (th, 1, &address); address = (char *) address + offset; } diff --git a/gdb/proc-service.c b/gdb/proc-service.c index 4620fea..940993c 100644 --- a/gdb/proc-service.c +++ b/gdb/proc-service.c @@ -102,6 +102,8 @@ ps_xfer_memory (const struct ps_prochandle *ph, psaddr_t addr, } +extern unsigned int libthread_db_debug; + /* Search for the symbol named NAME within the object named OBJ within the target process PH. If the symbol is found the address of the symbol is stored in SYM_ADDR. */ @@ -119,9 +121,20 @@ ps_pglobal_lookup (gdb_ps_prochandle_t ph, const char *obj, /* FIXME: kettenis/2000-09-03: What should we do with OBJ? */ bound_minimal_symbol ms = lookup_minimal_symbol (name, NULL, NULL); if (ms.minsym == NULL) - return PS_NOSYM; - - *sym_addr = core_addr_to_ps_addr (BMSYMBOL_VALUE_ADDRESS (ms)); + { + if (libthread_db_debug) + fprintf_unfiltered (gdb_stdlog, + "ps_pglobal_lookup: name=\"%s\" => PS_NOSYM\n", + name); + return PS_NOSYM; + } + + CORE_ADDR ms_addr = BMSYMBOL_VALUE_ADDRESS (ms); + if (libthread_db_debug) + fprintf_unfiltered (gdb_stdlog, + "ps_pglobal_lookup: name=\"%s\" => PS_OK, %s\n", name, + paddress (target_gdbarch (), ms_addr)); + *sym_addr = core_addr_to_ps_addr (ms_addr); return PS_OK; } -- 2.5.5
glibc.patch
(text/x-patch, 2.2 KB)
From 386b8dc8ef16197b3efa38f4bbbc98833ce7c2c6 Mon Sep 17 00:00:00 2001 From: Pedro Alves <[email protected]> Date: Mon, 11 Sep 2017 13:48:04 +0100 Subject: [PATCH] remove version checks hack --- nptl_db/td_ta_map_lwp2thr.c | 10 ++++++++-- nptl_db/td_ta_new.c | 9 ++++++++- 2 files changed, 16 insertions(+), 3 deletions(-) diff --git a/nptl_db/td_ta_map_lwp2thr.c b/nptl_db/td_ta_map_lwp2thr.c index d34711d..85620cb 100644 --- a/nptl_db/td_ta_map_lwp2thr.c +++ b/nptl_db/td_ta_map_lwp2thr.c @@ -185,11 +185,17 @@ td_ta_map_lwp2thr (const td_thragent_t *ta_arg, sometimes contain garbage that would confuse us, left by the kernel at exec. So if it looks like initialization is incomplete, we only fake a special descriptor for the initial thread. */ - psaddr_t list; td_err_e err = DB_GET_SYMBOL (list, ta, __stack_user); if (err != TD_OK) - return err; + { + /* '__stack_user' is in pthread.so, so this always fails with + non-threaded programs. GDB hardcodes/assumes th_unique==0 + for the main thread - maybe we should instead return the fake + special descriptor for the initial thread here too. See + below. */ + return err; + } err = DB_GET_FIELD (list, ta, list, list_t, next, 0); if (err != TD_OK) diff --git a/nptl_db/td_ta_new.c b/nptl_db/td_ta_new.c index aec2356..40424ad 100644 --- a/nptl_db/td_ta_new.c +++ b/nptl_db/td_ta_new.c @@ -33,12 +33,18 @@ LIST_HEAD (__td_agent_list); td_err_e td_ta_new (struct ps_prochandle *ps, td_thragent_t **ta) { +#if 0 psaddr_t versaddr; char versbuf[sizeof (VERSION)]; +#endif LOG ("td_ta_new"); - /* Check whether the versions match. */ + /* Check whether the versions match. + + XXX: Disabled because "nptl_version" currently lives in + libpthread.so. */ +#if 0 if (td_lookup (ps, SYM_nptl_version, &versaddr) != PS_OK) return TD_NOLIBTHREAD; if (ps_pdread (ps, versaddr, versbuf, sizeof (versbuf)) != PS_OK) @@ -47,6 +53,7 @@ td_ta_new (struct ps_prochandle *ps, td_thragent_t **ta) if (memcmp (versbuf, VERSION, sizeof VERSION) != 0) /* Not the right version. */ return TD_VERSION; +#endif /* Fill in the appropriate information. */ *ta = (td_thragent_t *) calloc (1, sizeof (td_thragent_t)); -- 2.5.5