[PATCH v6 03/11] Track and fetch TLS module ids for MUSL and GLIBC

[Kevin Buettner <kevinb@redhat.com>]

This commit adds, to solib-svr4.h and solib-svr4.c, functions
glibc_link_map_to_tls_module_id and musl_link_map_to_tls_module_id for
use with callers in a new file svr4-tls-tdep.c (which is not in this
commit).  It adds a number of helper functions for implementing link
map to module id support.

It also renames existing function 'find_program_interpreter' to
'svr4_find_program_interpreter' and makes it visible to other source
files within GDB.  It will be used in the libc sniffing code in
svr4-tls-tdep.c in a later commit in this series.  The libc sniffer is
needed in order to know which link map to module id function to call
as the method for determining module ids differs between libc /
dynamic linker implementations.  These details are discussed in
comments in the patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24548
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31563
---
 gdb/solib-svr4.c | 207 ++++++++++++++++++++++++++++++++++++++++++++++-
 gdb/solib-svr4.h |  12 +++
 2 files changed, 215 insertions(+), 4 deletions(-)

diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index 398123f7a52..99ba1bb8f6f 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -405,6 +405,9 @@ struct svr4_info
      The special entry zero is reserved for a linear list to support
      gdbstubs that do not support namespaces.  */
   std::map<CORE_ADDR, std::vector<svr4_so>> solib_lists;
+
+  bool glibc_tls_slots_inited = false;
+  std::vector<CORE_ADDR> glibc_tls_slots;
 };
 
 /* Per-program-space data key.  */
@@ -586,10 +589,10 @@ read_program_header (int type, int *p_arch_size, CORE_ADDR *base_addr)
   return buf;
 }
 
+/* See solib-svr4.h.  */
 
-/* Return program interpreter string.  */
-static std::optional<gdb::byte_vector>
-find_program_interpreter (void)
+std::optional<gdb::byte_vector>
+svr4_find_program_interpreter ()
 {
   /* If we have a current exec_bfd, use its section table.  */
   if (current_program_space->exec_bfd ()
@@ -1516,6 +1519,198 @@ svr4_fetch_objfile_link_map (struct objfile *objfile)
   return 0;
 }
 
+/* Return true if bfd section BFD_SECT is a thread local section
+   (i.e. either named ".tdata" or ".tbss"), and false otherwise.  */
+
+static bool
+is_thread_local_section (struct bfd_section *bfd_sect)
+{
+  return ((strcmp (bfd_sect->name, ".tdata") == 0
+	   || strcmp (bfd_sect->name, ".tbss") == 0)
+	  && bfd_sect->size != 0);
+}
+
+/* Return true if objfile OBJF contains a thread local section, and
+   false otherwise.  */
+
+static bool
+has_thread_local_section (const objfile *objf)
+{
+  for (obj_section *objsec : objf->sections ())
+    if (is_thread_local_section (objsec->the_bfd_section))
+      return true;
+  return false;
+}
+
+/* Return true if solib SO contains a thread local section, and false
+   otherwise.  */
+
+static bool
+has_thread_local_section (const solib &so)
+{
+  for (const target_section &p : so.sections)
+    if (is_thread_local_section (p.the_bfd_section))
+      return true;
+  return false;
+}
+
+/* For the MUSL C library, given link map address LM_ADDR, return the
+   corresponding TLS module id, or 0 if not found.
+   
+   Background: Unlike the mechanism used by glibc (see below), the
+   scheme used by the MUSL C library is pretty simple.  If the
+   executable contains TLS variables it gets module id 1.  Otherwise,
+   the first shared object loaded which contains TLS variables is
+   assigned to module id 1.  TLS-containing shared objects are then
+   assigned consecutive module ids, based on the order that they are
+   loaded.  When unloaded via dlclose, module ids are reassigned as if
+   that module had never been loaded.  */
+
+int
+musl_link_map_to_tls_module_id (CORE_ADDR lm_addr)
+{
+  /* When lm_addr is zero, the program is statically linked.  Any TLS
+     variables will be in module id 1.  */
+  if (lm_addr == 0)
+    return 1;
+
+  int mod_id = 0;
+  if (has_thread_local_section (current_program_space->symfile_object_file))
+    mod_id++;
+
+  struct svr4_info *info = get_svr4_info (current_program_space);
+
+  /* Cause svr4_current_sos() to be run if it hasn't been already.  */
+  if (info->main_lm_addr == 0)
+    solib_add (NULL, 0, auto_solib_add);
+
+  /* Handle case where lm_addr corresponds to the main program. 
+     Return value is either 0, when there are no TLS variables, or 1,
+     when there are.  */
+  if (lm_addr == info->main_lm_addr)
+    return mod_id;
+
+  /* Iterate through the shared objects, possibly incrementing the
+     module id, and returning mod_id should a match be found.  */
+  for (const solib &so : current_program_space->solibs ())
+    {
+      if (has_thread_local_section (so))
+	mod_id++;
+
+      auto *li = gdb::checked_static_cast<lm_info_svr4 *> (so.lm_info.get ());
+      if (li->lm_addr == lm_addr)
+	return mod_id;
+    }
+  return 0;
+}
+
+/* For GLIBC, given link map address LM_ADDR, return the corresponding TLS
+   module id, or 0 if not found.  */
+
+int
+glibc_link_map_to_tls_module_id (CORE_ADDR lm_addr)
+{
+  /* When lm_addr is zero, the program is statically linked.  Any TLS
+     variables will be in module id 1.  */
+  if (lm_addr == 0)
+    return 1;
+
+  /* Look up lm_addr in the TLS slot data structure.  */
+  struct svr4_info *info = get_svr4_info (current_program_space);
+  auto it = std::find (info->glibc_tls_slots.begin (),
+		       info->glibc_tls_slots.end (),
+		       lm_addr);
+  if (it == info->glibc_tls_slots.end ())
+    return 0;
+  else
+    return 1 + it - info->glibc_tls_slots.begin ();
+}
+
+/* Conditionally, based on whether the shared object, SO, contains TLS
+   variables, assign a link map address to a TLS module id slot.  This
+   code is GLIBC-specific and may only work for specific GLIBC
+   versions.  That said, it is known to work for (at least) GLIBC
+   versions 2.27 thru 2.40.
+
+   Background: In order to implement internal TLS address lookup
+   code, it is necessary to find the module id that has been
+   associated with a specific link map address.  In GLIBC, the TLS
+   module id is stored in struct link_map, in the member
+   'l_tls_modid'.  While the first several members of struct link_map
+   are part of the SVR4 ABI, the offset to l_tls_modid definitely is
+   not.  Therefore, since we don't know the offset to l_tls_modid, we
+   cannot simply look it up - which is a shame, because things would
+   be so much more easy and obviously accurate, if we could access
+   l_tls_modid.
+
+   GLIBC has a concept of TLS module id slots.  These slots are
+   allocated consecutively as shared objects containing TLS variables
+   are loaded.  When unloaded (e.g. via dlclose()), the corresponding
+   slot is marked as unused, but may be used again when later loading
+   a shared object.
+
+   The functions tls_maybe_fill_slot and tls_maybe_erase_slot are
+   associated with the observers 'solib_loaded' and 'solib_unloaded'. 
+   They (attempt to) track use of TLS module id slots in the same way
+   that GLIBC does, which will hopefully provide an accurate module id
+   when asked to provide it via glibc_link_map_to_tls_module_id(),
+   above.  */
+
+static void
+tls_maybe_fill_slot (solib &so)
+{
+  struct svr4_info *info = get_svr4_info (current_program_space);
+  if (!info->glibc_tls_slots_inited)
+    {
+      /* Cause svr4_current_sos() to be run if it hasn't been already.  */
+      if (info->main_lm_addr == 0)
+	svr4_current_sos_direct (info);
+
+      /* Quit early when main_lm_addr is still 0.  */
+      if (info->main_lm_addr == 0)
+	return;
+
+      /* Also quit early when symfile_object_file is not yet known.  */
+      if (current_program_space->symfile_object_file == nullptr)
+	return;
+
+      if (has_thread_local_section (current_program_space->symfile_object_file))
+	info->glibc_tls_slots.push_back (info->main_lm_addr);
+      info->glibc_tls_slots_inited = true;
+    }
+
+  if (has_thread_local_section (so))
+    {
+      auto it = std::find (info->glibc_tls_slots.begin (),
+			   info->glibc_tls_slots.end (),
+			   0);
+      auto *li = gdb::checked_static_cast<lm_info_svr4 *> (so.lm_info.get ());
+      if (it == info->glibc_tls_slots.end ())
+	info->glibc_tls_slots.push_back (li->lm_addr);
+      else
+	*it = li->lm_addr;
+    }
+}
+
+/* Remove a link map address from the TLS module slot data structure.
+   As noted above, this code is GLIBC-specific.  */
+
+static void
+tls_maybe_erase_slot (program_space *pspace, const solib &so,
+		      bool still_in_use, bool silent)
+{
+  if (still_in_use)
+    return;
+
+  struct svr4_info *info = get_svr4_info (pspace);
+  auto *li = gdb::checked_static_cast<lm_info_svr4 *> (so.lm_info.get ());
+  auto it = std::find (info->glibc_tls_slots.begin (),
+		       info->glibc_tls_slots.end (),
+		       li->lm_addr);
+  if (it != info->glibc_tls_slots.end ())
+    *it = 0;
+}
+
 /* On some systems, the only way to recognize the link map entry for
    the main executable file is by looking at its name.  Return
    non-zero iff SONAME matches one of the known main executable names.  */
@@ -2307,7 +2502,7 @@ enable_break (struct svr4_info *info, int from_tty)
   /* Find the program interpreter; if not found, warn the user and drop
      into the old breakpoint at symbol code.  */
   std::optional<gdb::byte_vector> interp_name_holder
-    = find_program_interpreter ();
+    = svr4_find_program_interpreter ();
   if (interp_name_holder)
     {
       const char *interp_name = (const char *) interp_name_holder->data ();
@@ -3483,4 +3678,8 @@ _initialize_svr4_solib ()
 {
   gdb::observers::free_objfile.attach (svr4_free_objfile_observer,
 				       "solib-svr4");
+
+  /* Set up observers for tracking GLIBC TLS module id slots.  */
+  gdb::observers::solib_loaded.attach (tls_maybe_fill_slot, "solib-svr4");
+  gdb::observers::solib_unloaded.attach (tls_maybe_erase_slot, "solib-svr4");
 }
diff --git a/gdb/solib-svr4.h b/gdb/solib-svr4.h
index 37cdaff4882..618eac52d72 100644
--- a/gdb/solib-svr4.h
+++ b/gdb/solib-svr4.h
@@ -112,4 +112,16 @@ extern struct link_map_offsets *svr4_lp64_fetch_link_map_offsets (void);
    SVR4 run time loader.  */
 int svr4_in_dynsym_resolve_code (CORE_ADDR pc);
 
+/* For the MUSL C library, given link map address LM_ADDR, return the
+   corresponding TLS module id, or 0 if not found.  */
+int musl_link_map_to_tls_module_id (CORE_ADDR lm_addr);
+
+/* For GLIBC, given link map address LM_ADDR, return the corresponding TLS
+   module id, or 0 if not found.  */
+int glibc_link_map_to_tls_module_id (CORE_ADDR lm_addr);
+
+/* Return program interpreter string.  */
+
+std::optional<gdb::byte_vector> svr4_find_program_interpreter ();
+
 #endif /* GDB_SOLIB_SVR4_H */
-- 
2.48.1