[binutils-gdb] gdb: attach to a process when the executable has been deleted

[binutils-gdb] gdb: attach to a process when the executable has been deleted

[Andrew Burgess]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=e58beedf2c8a1e0c78e0f57aeab3934de9416bfc

commit e58beedf2c8a1e0c78e0f57aeab3934de9416bfc
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Tue Jan 23 16:00:59 2024 +0000

    gdb: attach to a process when the executable has been deleted
    
    Bug PR gdb/28313 describes attaching to a process when the executable
    has been deleted.  The bug is for S390 and describes how a user sees a
    message 'PC not saved'.
    
    On x86-64 (GNU/Linux) I don't see a 'PC not saved' message, but
    instead I see this:
    
      (gdb) attach 901877
      Attaching to process 901877
      No executable file now.
      warning: Could not load vsyscall page because no executable was specified
      0x00007fa9d9c121e7 in ?? ()
      (gdb) bt
      #0  0x00007fa9d9c121e7 in ?? ()
      #1  0x00007fa9d9c1211e in ?? ()
      #2  0x0000000000000007 in ?? ()
      #3  0x000000002dc8b18d in ?? ()
      #4  0x0000000000000000 in ?? ()
      (gdb)
    
    Notice that the addresses in the backtrace don't seem right, quickly
    heading to 0x7 and finally ending at 0x0.
    
    What's going on, in both the s390 case and the x86-64 case is that the
    architecture's prologue scanner is going wrong and causing the stack
    unwinding to fail.
    
    The prologue scanner goes wrong because GDB has no unwind information.
    
    And GDB has no unwind information because, of course, the executable
    has been deleted.
    
    Notice in the example session above we get this line in the output:
    
      No executable file now.
    
    which indicates that GDB failed to find an executable to debug.
    
    For GNU/Linux when GDB tries to find an executable for a given pid we
    end up calling linux_proc_pid_to_exec_file in gdb/nat/linux-procfs.c.
    Within this function we call `readlink` on /proc/PID/exe to find the
    path of the actual executable.
    
    If the `readlink` call fails then we already fallback on using
    /proc/PID/exe as the path to the executable to debug.
    
    However, when the executable has been deleted the `readlink` call
    doesn't fail, but the path that is returned points to a non-existent
    file.
    
    I propose that we add an `access` call to linux_proc_pid_to_exec_file
    to check that the target file exists and can be read.  If the target
    can't be read then we should fall back to /proc/PID/exe (assuming that
    /proc/PID/exe can be read).
    
    Now on x86-64 the output looks like this:
    
      (gdb) attach 901877
      Attaching to process 901877
      Reading symbols from /proc/901877/exe...
      Reading symbols from /lib64/libc.so.6...
      (No debugging symbols found in /lib64/libc.so.6)
      Reading symbols from /lib64/ld-linux-x86-64.so.2...
      (No debugging symbols found in /lib64/ld-linux-x86-64.so.2)
      0x00007fa9d9c121e7 in nanosleep () from /lib64/libc.so.6
      (gdb) bt
      #0  0x00007fa9d9c121e7 in nanosleep () from /lib64/libc.so.6
      #1  0x00007fa9d9c1211e in sleep () from /lib64/libc.so.6
      #2  0x000000000040117e in spin_forever () at attach-test.c:17
      #3  0x0000000000401198 in main () at attach-test.c:24
      (gdb)
    
    which is much better.
    
    I've also tagged the bug PR gdb/29782 which concerns the test
    gdb.server/connect-with-no-symbol-file.exp.  After making this change,
    when running gdb.server/connect-with-no-symbol-file.exp GDB would now
    pick up the /proc/PID/exe file as the executable in some cases.
    
    As GDB is not restarted for the multiple iterations of this test
    GDB (or rather BFD) would given a warning/error like:
    
      (gdb) PASS: gdb.server/connect-with-no-symbol-file.exp: sysroot=target:: action=permission: setup: disconnect
      set sysroot target:
      BFD: reopening /proc/3283001/exe: No such file or directory
      (gdb) FAIL: gdb.server/connect-with-no-symbol-file.exp: sysroot=target:: action=permission: setup: adjust sysroot
    
    What's happening is that an executable found for an earlier iteration
    of the test is still registered for the inferior when we are setting
    up for a second iteration of the test.  When the sysroot changes, if
    there's an executable registered GDB tries to reopen it, but in this
    case the file has disappeared (the previous inferior has exited by
    this point).
    
    I did think about maybe, when the executable is /proc/PID/exe, we
    should auto-delete the file from the inferior.  But in the end I
    thought this was a bad idea.  Not only would this require a lot of
    special code in GDB just to support this edge case: we'd need to track
    if the exe file name came from /proc and should be auto-deleted, or
    we'd need target specific code to check if a path should be
    auto-deleted.....
    
    ... in addition, we'd still want to warn the user when we auto-deleted
    the file from the inferior, otherwise they might be surprised to find
    their inferior suddenly has no executable attached, so we wouldn't
    actually reduce the number of warnings the user sees.
    
    So in the end I figured that the best solution is to just update the
    test to avoid the warning.  This is easily done by manually removing
    the executable from the inferior once each iteration of the test has
    completed.
    
    Now, in bug PR gdb/29782 GDB is clearly managing to pick up an
    executable from the NFS cache somehow.  I guess what's happening is
    that when the original file is deleted /proc/PID/exe is actually
    pointing to a file in the NFS cache which is only deleted at some
    later point, and so when GDB starts up we do manage to associate a
    file with the inferior, this results in the same message being emitted
    from BFD as I was seeing.  The fix included in this commit should also
    fix that bug.
    
    One final note:  On x86-64 GNU/Linux, the
    gdb.server/connect-with-no-symbol-file.exp test will produce 2 core
    files.  This is due to a bug in gdbserver that is nothing to do with
    this test.  These core files are created before and after this
    commit.  I am working on a fix for the gdbserver issue, but will post
    that separately.
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28313
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29782
    
    Approved-By: Tom Tromey <tom@tromey.com>

Diff:
---
 gdb/nat/linux-procfs.c                             |  5 ++
 gdb/testsuite/gdb.base/attach-deleted-exec.c       | 29 ++++++++++++
 gdb/testsuite/gdb.base/attach-deleted-exec.exp     | 53 ++++++++++++++++++++++
 .../gdb.server/connect-with-no-symbol-file.exp     | 12 +++++
 4 files changed, 99 insertions(+)

diff --git a/gdb/nat/linux-procfs.c b/gdb/nat/linux-procfs.c
index 6be3a26f252..b17e3120792 100644
--- a/gdb/nat/linux-procfs.c
+++ b/gdb/nat/linux-procfs.c
@@ -352,6 +352,11 @@ linux_proc_pid_to_exec_file (int pid)
   else
     buf[len] = '\0';
 
+  /* Use /proc/PID/exe if the actual file can't be read, but /proc/PID/exe
+     can be.  */
+  if (access (buf, R_OK) != 0 && access (name, R_OK) == 0)
+    strcpy (buf, name);
+
   return buf;
 }
 
diff --git a/gdb/testsuite/gdb.base/attach-deleted-exec.c b/gdb/testsuite/gdb.base/attach-deleted-exec.c
new file mode 100644
index 00000000000..ebfae87fbfb
--- /dev/null
+++ b/gdb/testsuite/gdb.base/attach-deleted-exec.c
@@ -0,0 +1,29 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2024 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+
+int
+main ()
+{
+  alarm (60);
+
+  while (1)
+    usleep (100000);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/attach-deleted-exec.exp b/gdb/testsuite/gdb.base/attach-deleted-exec.exp
new file mode 100644
index 00000000000..3e31c36bcc4
--- /dev/null
+++ b/gdb/testsuite/gdb.base/attach-deleted-exec.exp
@@ -0,0 +1,53 @@
+# Copyright (C) 2024 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Attach to a process, the executable for which has been deleted.  On
+# GNU/Linux GDB will spot the missing executable and fallback to use
+# /proc/PID/exe instead.
+
+require can_spawn_for_attach
+require {istarget *-linux*}
+
+standard_testfile
+
+if { [build_executable "failed to prepare" $testfile $srcfile] } {
+    return -1
+}
+
+set test_spawn_id [spawn_wait_for_attach $binfile]
+set testpid [spawn_id_get_pid $test_spawn_id]
+
+# Move the executable rather than deleting it.  This just to aid with
+# debugging if someone needs to reproduce this test.
+set binfile_moved ${binfile}_moved
+
+# Don't move BINFILE as the kernel will just assign a new name to the
+# same inode; and the /proc/PID/exe link will continue to point to the
+# renamed inode.
+remote_exec host "cp $binfile $binfile_moved"
+remote_exec host "rm $binfile"
+
+# Don't pass the executable when GDB starts.  Instead rely on GDB
+# finding the executable from the PID we attach too.
+clean_restart
+
+# Attach.  GDB should spot that the executable is gone and fallback to
+# use /proc/PID/exe.
+gdb_test "attach $testpid" \
+    "Attaching to process $decimal\r\nReading symbols from /proc/${testpid}/exe\\.\\.\\..*" \
+    "attach to process with deleted executable"
+
+# Cleanup.
+kill_wait_spawned_process $test_spawn_id
diff --git a/gdb/testsuite/gdb.server/connect-with-no-symbol-file.exp b/gdb/testsuite/gdb.server/connect-with-no-symbol-file.exp
index 284004abadd..becd94d9f90 100644
--- a/gdb/testsuite/gdb.server/connect-with-no-symbol-file.exp
+++ b/gdb/testsuite/gdb.server/connect-with-no-symbol-file.exp
@@ -89,6 +89,18 @@ proc connect_no_symbol_file { sysroot action } {
 	}
     }
     gdb_assert $ok "connection to GDBserver succeeded"
+
+    # GDB will register /proc/PID/exe as the executable for some of
+    # these tests.  Once the test has finished the inferior will still
+    # have /proc/PID/exe registered as its executable even though that
+    # file no longer exists (most likely).  GDB will then complain
+    # about the inferior's executable having disappeared.  Silence
+    # these warnings by removing any registered file from the
+    # executable.
+    gdb_test "with confirm off -- file" \
+	[multi_line \
+	     "No executable file now\\." \
+	     "No symbol file now\\."]
 }
 
 # Make sure we have the original symbol file in a safe place to copy from.