[PATCH v1 02/10] gdb: rely on the first alive thread TPID when reading Linux procfs files

[Matthieu Longo <matthieu.longo@arm.com>]

On Linux, /proc/<pid> is keyed by the thread-group leader PID. When
the leader has exited, some /proc/<pid>/... entries become unavailable
even though another thread is still alive. This can happen, for instance,
when the main thread calls pthread_exit() and another thread continues
the execution (existing test: gcore-stale-thread).

This causes GDB to fail to read procfs entries such as cmdline, cwd,
exe, maps, and smaps when it uses 'current_inferior ()->pid' after the
thread-group leader has exited.

Fix this by adding inferior::first_alive_thread(), which returns the
PTID of the first non-exited thread of the inferior. Use its LWP ID
when accessing procfs entries that only need a representative live LWP
belonging to the process.

Update linux_info_proc, linux_process_address_in_memtag_page, and
linux_find_memory_regions_full to use this live-thread LWP ID instead
of the inferior PID when constructing procfs paths.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31207
---
 gdb/inferior.c   | 12 ++++++++++++
 gdb/inferior.h   |  9 +++++++++
 gdb/linux-tdep.c | 38 ++++++++++++++++++--------------------
 3 files changed, 39 insertions(+), 20 deletions(-)

diff --git a/gdb/inferior.c b/gdb/inferior.c
index 1481f46cdd1..3b6ec60fb1b 100644
--- a/gdb/inferior.c
+++ b/gdb/inferior.c
@@ -250,6 +250,18 @@ inferior::find_thread (ptid_t ptid)
 
 /* See inferior.h.  */
 
+ptid_t
+inferior::first_alive_thread () const
+{
+  auto it = this->ptid_thread_map.cbegin ();
+  if (it != this->ptid_thread_map.cend ())
+    return it->first;
+  else
+    return ptid_t::make_null ();
+}
+
+/* See inferior.h.  */
+
 void
 inferior::clear_thread_list ()
 {
diff --git a/gdb/inferior.h b/gdb/inferior.h
index 9c031035a23..305b1d31830 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -513,6 +513,15 @@ class inferior : public refcounted_object,
   /* Find (non-exited) thread PTID of this inferior.  */
   thread_info *find_thread (ptid_t ptid);
 
+  /* Return the first (non-exited) thread PTID of this inferior.
+
+     This method should be used in place of current_inferior ()->pid for any
+     features relying only on the PID like the reading of procfs files.
+     On Linux, /proc/<pid> is keyed by the thread-group leader PID. When the
+     leader has exited, some /proc/<pid>/... entries become unavailable even
+     though another thread is still alive.  */
+  ptid_t first_alive_thread () const;
+
   /* Delete all threads in the thread list, silently.  */
   void clear_thread_list ();
 
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 155d6e874d9..b89f4ee0717 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -842,9 +842,7 @@ static void
 linux_info_proc (struct gdbarch *gdbarch, const char *args,
 		 enum info_proc_what what)
 {
-  /* A long is used for pid instead of an int to avoid a loss of precision
-     compiler warning from the output of strtoul.  */
-  long pid;
+  ptid_t ptid;
   int cmdline_f = (what == IP_MINIMAL || what == IP_CMDLINE || what == IP_ALL);
   int cwd_f = (what == IP_MINIMAL || what == IP_CWD || what == IP_ALL);
   int environ_f = (what == IP_ENVIRON || what == IP_ALL);
@@ -859,7 +857,8 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     {
       char *tem;
 
-      pid = strtoul (args, &tem, 10);
+      auto pid = strtoul (args, &tem, 10);
+      ptid = ptid_t (pid, pid);
       args = tem;
     }
   else
@@ -869,17 +868,17 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
       if (current_inferior ()->fake_pid_p)
 	error (_("Can't determine the current process's PID: you must name one."));
 
-      pid = current_inferior ()->pid;
+      ptid = current_inferior ()->first_alive_thread ();
     }
 
   args = skip_spaces (args);
   if (args && args[0])
     error (_("Too many parameters: %s"), args);
 
-  gdb_printf (_("process %ld\n"), pid);
+  gdb_printf (_("process %d\n"), ptid.pid ());
   if (cmdline_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/cmdline", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/cmdline", ptid.lwp ());
       gdb_byte *buffer;
       LONGEST len = target_fileio_read_alloc (nullptr, filename, &buffer);
 
@@ -901,7 +900,7 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     }
   if (cwd_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/cwd", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/cwd", ptid.lwp ());
       std::optional<std::string> contents
 	= target_fileio_readlink (NULL, filename, &target_errno);
       if (contents.has_value ())
@@ -911,7 +910,7 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     }
   if (environ_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/environ", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/environ", ptid.lwp ());
       gdb_byte *buffer;
       LONGEST len = target_fileio_read_alloc (nullptr, filename, &buffer);
 
@@ -935,7 +934,7 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     }
   if (exe_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/exe", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/exe", ptid.lwp ());
       std::optional<std::string> contents
 	= target_fileio_readlink (NULL, filename, &target_errno);
       if (contents.has_value ())
@@ -945,7 +944,7 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     }
   if (mappings_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/maps", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/maps", ptid.lwp ());
       gdb::unique_xmalloc_ptr<char> map
 	= target_fileio_read_stralloc (NULL, filename);
       if (map != NULL)
@@ -990,7 +989,7 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     }
   if (status_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/status", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/status", ptid.lwp ());
       gdb::unique_xmalloc_ptr<char> status
 	= target_fileio_read_stralloc (NULL, filename);
       if (status)
@@ -1000,7 +999,7 @@ linux_info_proc (struct gdbarch *gdbarch, const char *args,
     }
   if (stat_f)
     {
-      xsnprintf (filename, sizeof filename, "/proc/%ld/stat", pid);
+      xsnprintf (filename, sizeof filename, "/proc/%ld/stat", ptid.lwp ());
       gdb::unique_xmalloc_ptr<char> statstr
 	= target_fileio_read_stralloc (NULL, filename);
       if (statstr)
@@ -1671,9 +1670,9 @@ linux_process_address_in_memtag_page (CORE_ADDR address)
   if (current_inferior ()->fake_pid_p)
     return false;
 
-  pid_t pid = current_inferior ()->pid;
+  ptid_t ptid = current_inferior ()->first_alive_thread ();
 
-  std::string smaps_file = string_printf ("/proc/%d/smaps", pid);
+  std::string smaps_file = string_printf ("/proc/%ld/smaps", ptid.lwp ());
 
   gdb::unique_xmalloc_ptr<char> data
     = target_fileio_read_stralloc (NULL, smaps_file.c_str ());
@@ -1731,7 +1730,6 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
 				linux_dump_mapping_p_ftype *should_dump_mapping_p,
 				linux_find_memory_region_ftype func)
 {
-  pid_t pid;
   /* Default dump behavior of coredump_filter (0x33), according to
      Documentation/filesystems/proc.txt from the Linux kernel
      tree.  */
@@ -1744,12 +1742,12 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
   if (current_inferior ()->fake_pid_p)
     return false;
 
-  pid = current_inferior ()->pid;
+  ptid_t ptid = current_inferior ()->first_alive_thread ();
 
   if (use_coredump_filter)
     {
       std::string core_dump_filter_name
-	= string_printf ("/proc/%d/coredump_filter", pid);
+	= string_printf ("/proc/%ld/coredump_filter", ptid.lwp ());
 
       gdb::unique_xmalloc_ptr<char> coredumpfilterdata
 	= target_fileio_read_stralloc (NULL, core_dump_filter_name.c_str ());
@@ -1763,7 +1761,7 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
 	}
     }
 
-  std::string maps_filename = string_printf ("/proc/%d/smaps", pid);
+  std::string maps_filename = string_printf ("/proc/%ld/smaps", ptid.lwp ());
 
   gdb::unique_xmalloc_ptr<char> data
     = target_fileio_read_stralloc (NULL, maps_filename.c_str ());
@@ -1771,7 +1769,7 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
   if (data == NULL)
     {
       /* Older Linux kernels did not support /proc/PID/smaps.  */
-      maps_filename = string_printf ("/proc/%d/maps", pid);
+      maps_filename = string_printf ("/proc/%ld/maps", ptid.lwp ());
       data = target_fileio_read_stralloc (NULL, maps_filename.c_str ());
 
       if (data == nullptr)
-- 
2.55.0