Using libthread_db.so with single-threaded programs, for TLS access (was: Re: [RFC PATCH 0/3] Pretty-printing for errno)

Pedro Alves
2017-09-13T11:22:59Z

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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

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From 24f816f528fa110b2dc670a5f43899b60983ac2b Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
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,
}
\f

+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

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From 386b8dc8ef16197b3efa38f4bbbc98833ce7c2c6 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
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