[PATCHv5] gdb: handle loading shared libraries from /proc/self/fd/

  Asaf pointed out to me (off list) that v4 fails for some cases that the
original patch supported, so here's a v5.  Specifically, if the dlopen
call is passed a non-canonical path then GDB will fail to correctly spot
that the file being opened is within /proc/self, and will still try to
open a file within GDB's /proc/self by mistake.

Changes since v4:

  - filter_proc_self_filenames now makes use of gdb_realpath_keepfile to
    (partially) canonicalize the path, this should fix the non-canonical
    path issues,

  - extended the test to include the non-canonical path case.

Changes since v3:

  - Fixed NULL/nullptr issues Lancelot pointed out,

  - Updated the test case to take account of non-probes based shared
    library event handling.

---

commit 65c1f020184e632a6de1775c00de7e0d7a97def3
Author: Asaf Fisher via Gdb-patches <gdb-patches@sourceware.org>
Date:   Fri Oct 21 17:42:05 2022 +0000

    gdb: handle loading shared libraries from /proc/self/fd/
    
    Bug PR gdb/29586 describes a situations where a shared library is
    created in memory, then written to a memory mapped file.  The memory
    mapped file will show up as a file descriptor within /proc/self/fd/,
    and this path is then used with dlopen in order to call functions
    within the in-memory shared library.
    
    When attempting to debug this GDB hangs.  The problem is that, GDB
    stops at the shared-library event, reads the shared library path from
    the inferior, which is "/proc/self/fd/<NUM>", and then GDB attempts to
    open this file.
    
    Unfortunately, this means GDB tries to open a file within GDB's
    /proc/self/fd/ directory, not within the inferior's directory.  In the
    case of our hang it turns out that the file descriptor that is opened
    is a pipe, and GDB hangs trying to read from the pipe.
    
    However, the behaviour is really just undefined, depending on which
    file descriptor the inferior tries to open, GDB will open, or fail to
    open, random files within its /proc/self/fd directory.
    
    The solution proposed in this commit is to hook into solib_find_1, and
    spot when GDB is looking for any file in /proc/self/, if this is the
    case, then the filename is rewritten as /proc/<PID>, where <PID> is
    the process-id of the current inferior.
    
    One complexity that we need to consider is how to handle non-canonical
    paths, for example, if the user opens /proc/../proc/self/fd/<NUM> then
    GDB also needs to be able to spot this.
    
    My solution is to call gdb_realpath_keepfile, this canonicalizes
    everything except the filename (the '/proc/../proc/self/fd' part in
    the above example).  The problem this then introduces is that the
    'self' is replaces with the pid of GDB itself.  I therefore check,
    after canonicalization, for a prefix of /proc/<PID OF GDB>, and if
    this is the case, I replace this with /proc/<PID OF INFERIOR>.
    
    There is one remaining issue, which I consider very unlikely, and I
    don't propose to fix.  What if the inferior actually did try to dlopen
    a file from /proc/<PID OF GDB>?  If this happened then GDB will not be
    able to distinguish this from the above case and would still replace
    the pid of GDB with the pid of the inferior.  However, this seems
    really unlikely - why would the inferior be trying to load parts of
    GDB?  So I propose to ignore this case until someone can come up with
    an actual scenario where that's useful.
    
    The test passes for the unix, native-gdbserver, and
    native-extended-gdbserver targets.  When testing with either of the
    gdbserver targets, the test is run using the default empty sysroot,
    and also using the 'target:' sysroot.
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29586
    
    Co-authored-by: Andrew Burgess <aburgess@redhat.com>
  

Looks good to me!

On Thu, Dec 15, 2022 at 6:44 PM Andrew Burgess <aburgess@redhat.com> wrote:

>
> Asaf pointed out to me (off list) that v4 fails for some cases that the
> original patch supported, so here's a v5.  Specifically, if the dlopen
> call is passed a non-canonical path then GDB will fail to correctly spot
> that the file being opened is within /proc/self, and will still try to
> open a file within GDB's /proc/self by mistake.
>
> Changes since v4:
>
>   - filter_proc_self_filenames now makes use of gdb_realpath_keepfile to
>     (partially) canonicalize the path, this should fix the non-canonical
>     path issues,
>
>   - extended the test to include the non-canonical path case.
>
> Changes since v3:
>
>   - Fixed NULL/nullptr issues Lancelot pointed out,
>
>   - Updated the test case to take account of non-probes based shared
>     library event handling.
>
> ---
>
> commit 65c1f020184e632a6de1775c00de7e0d7a97def3
> Author: Asaf Fisher via Gdb-patches <gdb-patches@sourceware.org>
> Date:   Fri Oct 21 17:42:05 2022 +0000
>
>     gdb: handle loading shared libraries from /proc/self/fd/
>
>     Bug PR gdb/29586 describes a situations where a shared library is
>     created in memory, then written to a memory mapped file.  The memory
>     mapped file will show up as a file descriptor within /proc/self/fd/,
>     and this path is then used with dlopen in order to call functions
>     within the in-memory shared library.
>
>     When attempting to debug this GDB hangs.  The problem is that, GDB
>     stops at the shared-library event, reads the shared library path from
>     the inferior, which is "/proc/self/fd/<NUM>", and then GDB attempts to
>     open this file.
>
>     Unfortunately, this means GDB tries to open a file within GDB's
>     /proc/self/fd/ directory, not within the inferior's directory.  In the
>     case of our hang it turns out that the file descriptor that is opened
>     is a pipe, and GDB hangs trying to read from the pipe.
>
>     However, the behaviour is really just undefined, depending on which
>     file descriptor the inferior tries to open, GDB will open, or fail to
>     open, random files within its /proc/self/fd directory.
>
>     The solution proposed in this commit is to hook into solib_find_1, and
>     spot when GDB is looking for any file in /proc/self/, if this is the
>     case, then the filename is rewritten as /proc/<PID>, where <PID> is
>     the process-id of the current inferior.
>
>     One complexity that we need to consider is how to handle non-canonical
>     paths, for example, if the user opens /proc/../proc/self/fd/<NUM> then
>     GDB also needs to be able to spot this.
>
>     My solution is to call gdb_realpath_keepfile, this canonicalizes
>     everything except the filename (the '/proc/../proc/self/fd' part in
>     the above example).  The problem this then introduces is that the
>     'self' is replaces with the pid of GDB itself.  I therefore check,
>     after canonicalization, for a prefix of /proc/<PID OF GDB>, and if
>     this is the case, I replace this with /proc/<PID OF INFERIOR>.
>
>     There is one remaining issue, which I consider very unlikely, and I
>     don't propose to fix.  What if the inferior actually did try to dlopen
>     a file from /proc/<PID OF GDB>?  If this happened then GDB will not be
>     able to distinguish this from the above case and would still replace
>     the pid of GDB with the pid of the inferior.  However, this seems
>     really unlikely - why would the inferior be trying to load parts of
>     GDB?  So I propose to ignore this case until someone can come up with
>     an actual scenario where that's useful.
>
>     The test passes for the unix, native-gdbserver, and
>     native-extended-gdbserver targets.  When testing with either of the
>     gdbserver targets, the test is run using the default empty sysroot,
>     and also using the 'target:' sysroot.
>
>     Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29586
>
>     Co-authored-by: Andrew Burgess <aburgess@redhat.com>
>
> diff --git a/gdb/solib.c b/gdb/solib.c
> index 59fd866b652..a627d217c85 100644
> --- a/gdb/solib.c
> +++ b/gdb/solib.c
> @@ -50,6 +50,7 @@
>  #include "gdb_bfd.h"
>  #include "gdbsupport/filestuff.h"
>  #include "gdbsupport/scoped_fd.h"
> +#include "gdbsupport/pathstuff.h"
>  #include "debuginfod-support.h"
>  #include "source.h"
>  #include "cli/cli-style.h"
> @@ -79,6 +80,54 @@ show_solib_search_path (struct ui_file *file, int
> from_tty,
>  #  define DOS_BASED_FILE_SYSTEM 0
>  #endif
>
> +/* Fix references to files in /proc/self/fd/ when opening a shared
> library.
> +
> +   SO_NAME is the name of the shared library being loaded.  This function
> +   returns a possibly modified name which should be used as the path to
> the
> +   shared library.
> +
> +   If SO_NAME starts with /proc/self, then the returned name will be
> +   modified to start with /proc/PID where 'PID' is the pid of the current
> +   inferior.  */
> +
> +static gdb::unique_xmalloc_ptr<char>
> +filter_proc_self_filenames (gdb::unique_xmalloc_ptr<char> so_name)
> +{
> +  /* In order to figure out if SO_NAME points at a file in /proc/self we
> +     need to canonicalize the path.  However, the whole point here is that
> +     if SO_NAME does point at /proc/self then GDB will think this is its
> +     self, while the inferior actually means its /proc/self.  A result of
> +     this is that the file being referenced within the inferior's
> +     /proc/self, might not exist within GDB's /proc/self, a call to
> +     gdb_realpath would then fail.
> +
> +     To avoid this problem we call gdb_realpath_keepfile, this only tries
> +     to canonicalize the basename of SO_NAME, and leaves the final
> filename
> +     untouched.  */
> +  so_name
> +    = make_unique_xstrdup (gdb_realpath_keepfile (so_name.get ()).c_str
> ());
> +
> +  /* A result of the above canonicalization is that /proc/self will have
> +     been replaced with /proc/<pid of gdb>, so that's what we need to
> check
> +     for.  Of course, it could be the case that the inferior really did
> try
> +     to dlopen a file within GDB's /proc/<pid> directory, in which case
> +     we're going to do the wrong thing here, but that seems far less
> likely
> +     than calling dlopen on a file within the inferior's own directory.
> */
> +  std::string prefix = string_printf ("/proc/%ld", (long) getpid ());
> +
> +  /* Is the canonical path inside GDB's /proc/<pid> directory?  */
> +  if (!startswith (so_name.get (), prefix.c_str ()))
> +    return so_name;
> +
> +  /* Get the part of the path after /proc/<pid>.  For example given
> +     '/proc/123/fd' we find the '/fd' part.  */
> +  const char *tail = so_name.get () + strlen (prefix.c_str ());
> +
> +  /* Build a replacement path within the inferiors directory.  */
> +  int inferior_pid = inferior_ptid.pid ();
> +  return xstrprintf ("/proc/%d%s", inferior_pid, tail);
> +}
> +
>  /* Return the full pathname of a binary file (the main executable or a
>     shared library file), or NULL if not found.  If FD is non-NULL, *FD
>     is set to either -1 or an open file handle for the binary file.
> @@ -168,6 +217,9 @@ solib_find_1 (const char *in_pathname, int *fd, bool
> is_solib)
>         }
>      }
>
> +  temp_pathname.reset (xstrdup (in_pathname));
> +  temp_pathname = filter_proc_self_filenames (std::move (temp_pathname));
> +
>    /* Note, we're interested in IS_TARGET_ABSOLUTE_PATH, not
>       IS_ABSOLUTE_PATH.  The latter is for host paths only, while
>       IN_PATHNAME is a target path.  For example, if we're supposed to
> @@ -180,9 +232,7 @@ solib_find_1 (const char *in_pathname, int *fd, bool
> is_solib)
>         3rd attempt, c:/foo/bar.dll ==> /sysroot/foo/bar.dll
>    */
>
> -  if (!IS_TARGET_ABSOLUTE_PATH (fskind, in_pathname) || sysroot == NULL)
> -    temp_pathname.reset (xstrdup (in_pathname));
> -  else
> +  if (IS_TARGET_ABSOLUTE_PATH (fskind, in_pathname) && sysroot != nullptr)
>      {
>        bool need_dir_separator;
>
> @@ -209,7 +259,7 @@ solib_find_1 (const char *in_pathname, int *fd, bool
> is_solib)
>        /* Cat the prefixed pathname together.  */
>        temp_pathname.reset (concat (sysroot,
>                                    need_dir_separator ? SLASH_STRING : "",
> -                                  in_pathname, (char *) NULL));
> +                                  temp_pathname.get (), nullptr));
>      }
>
>    /* Handle files to be accessed via the target.  */
> diff --git a/gdb/testsuite/gdb.base/solib-proc-self.cc
> b/gdb/testsuite/gdb.base/solib-proc-self.cc
> new file mode 100644
> index 00000000000..aea397a66f9
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/solib-proc-self.cc
> @@ -0,0 +1,89 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   Copyright 2022 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 <sys/mman.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <iostream>
> +#include <fstream>
> +#include <sstream>
> +#include <vector>
> +#include <unistd.h>
> +
> +#ifdef __WIN32__
> +#include <windows.h>
> +#define dlopen(name, mode) LoadLibrary (name)
> +#define dlclose(handle) FreeLibrary (handle)
> +#define dlerror() "an error occurred"
> +#else
> +#include <dlfcn.h>
> +#endif
> +
> +/* Combine PREFIX and NUM into a single string, joined with a '/', and
> +   return the string.  */
> +
> +std::string
> +make_library_path (const char *prefix, int num)
> +{
> +  std::stringstream stream;
> +  stream << prefix << "/" << num;
> +  return stream.str ();
> +}
> +
> +/* Call dlopen on the library pointed to by FILENAME.  */
> +
> +void
> +open_library (const std::string &filename)
> +{
> +  /* Call dlopen on FILENAME.  */
> +  void *handle = dlopen (filename.c_str (), RTLD_LAZY);
> +  if (handle == nullptr)
> +    abort ();
> +
> +  /* It worked.  */
> +  dlclose (handle);
> +}
> +
> +int
> +main ()
> +{
> +  /* Read the shared libraries contents into a buffer.  */
> +  std::ifstream read_so_file = std::ifstream (SHLIB_NAME);
> +  read_so_file.seekg (0, std::ios::end);
> +  std::streamsize size = read_so_file.tellg ();
> +  read_so_file.seekg (0, std::ios::beg);
> +  std::vector<char> buffer (size);
> +  if (!read_so_file.read (buffer.data (), size))
> +    abort ();
> +
> +  /* Create a memory mapped file, then write the shared library to that
> +     new memory mapped file.  */
> +  int mem_fd = memfd_create ("test", 0);
> +  write (mem_fd, buffer.data (), buffer.size ());
> +
> +  /* Generate a canonical /proc/self/fd/[num] path for the memory mapped
> +     file, and call dlopen on it.  */
> +  std::string filename
> +    = make_library_path ("/proc/self/fd", mem_fd);     /* break-here */
> +  open_library (filename);
> +
> +  /* Now generate a new, non-canonical filename, and call dlopen on it.
> */
> +  filename = make_library_path ("/proc/../proc/self/fd", mem_fd);
> +  open_library (filename);
> +
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.base/solib-proc-self.exp
> b/gdb/testsuite/gdb.base/solib-proc-self.exp
> new file mode 100644
> index 00000000000..01e7eecfb16
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/solib-proc-self.exp
> @@ -0,0 +1,200 @@
> +# Copyright 2022 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/>.
> */
> +
> +# Test connecting and disconnecting at shared library events.
> +
> +if {[skip_shlib_tests]} {
> +    untested "could not run to main"
> +    return 0
> +}
> +
> +standard_testfile .cc
> +
> +# Reuse an existing library, we don't care about the library contents
> +# for this test.
> +set libfile so-disc-shr
> +set libsrc "${srcdir}/${subdir}/${libfile}.c"
> +set libname "${libfile}.so"
> +set libobj [standard_output_file ${libname}]
> +
> +# Compile the shared library.
> +if { [gdb_compile_shlib $libsrc $libobj {debug}] != ""} {
> +    return -1
> +}
> +
> +# Compile the test executable.
> +if [ build_executable "failed to prepare" $testfile $srcfile \
> +        [list shlib_load debug c++
> additional_flags=-DSHLIB_NAME="${libobj}"]] {
> +    return -1
> +}
> +
> +proc validate_library_load {inferior_pid memfd} {
> +
> +    # Turn on the solib-events so we can see that gdb resolves
> +    # everything correctly.
> +    gdb_test_no_output "set stop-on-solib-events 1"
> +
> +    # Now continue forward until the solib event is detected, and
> +    # check that the loaded library is found through the /proc/PID/fd
> +    # rather than /proc/self/fd.
> +    #
> +    # We need to handle the possibility of the interesting event
> +    # showing the first, or second time we stop, as depending on which
> +    # mechanism GDB is using to handle the shared library events (the
> +    # newer probes based interface, or the old non-probes interface),
> +    # the library will be reported at the first or second stop.
> +    set saw_expected_event false
> +    set saw_no_event_stop false
> +    gdb_test_multiple "continue" "continue to solib evnt" {
> +       -re "^continue\r\n" {
> +           exp_continue
> +       }
> +
> +       -re "^Continuing\\.\r\n" {
> +           exp_continue
> +       }
> +
> +       -early -re "Stopped due to shared library event \\(no libraries
> added or removed\\)\r\n" {
> +           # This non-interesting event shows up first when using the
> +           # probes based mechanism for dealing with shared library
> +           # events.
> +           #
> +           # We set a flag here, and, once the prompt has appeared,
> +           # we send another continue, the next event will contain
> +           # the information we want.
> +           set saw_no_event_stop true
> +           exp_continue
> +       }
> +
> +       -re "Stopped due to shared library event:\r\n  Inferior loaded
> (?:target:)?/proc/${inferior_pid}/fd/$memfd\r\n" {
> +           # This event, which includes the information we are
> +           # looking for, occurs first when using the non-probes
> +           # based mechanism for handling shared library events, and
> +           # occurs second when using the probes mechanism.
> +           #
> +           # Either way, record here that we say the output we expect.
> +           set saw_expected_event true
> +           exp_continue
> +       }
> +
> +       -re "$::gdb_prompt $" {
> +           if {$saw_no_event_stop} {
> +               set saw_no_event_stop false
> +               send_gdb "continue\n"
> +               exp_continue
> +           } else {
> +               gdb_assert {$saw_expected_event} $gdb_test_name
> +           }
> +       }
> +    }
> +
> +    # Turn off solib events.  We're only interested in validating the
> +    # loads for now.
> +    gdb_test_no_output "set stop-on-solib-events 0"
> +}
> +
> +# Start GDB and run to the point where the test program tries to dlopen a
> file
> +# from within /proc/self/fd/.  Catch the shared library event and check
> that
> +# we actually try to load a file from /proc/<INFERIOR-PID>/fd/.
> +#
> +# If SYSROOT is not the empty string, then this is set as the value of
> GDB's
> +# sysroot immediately after starting GDB.  The only value that is
> (currently)
> +# supported, other than the empty string, is 'target:'.
> +proc do_test { {sysroot ""} } {
> +    clean_restart $::binfile
> +
> +    if {$sysroot != ""} {
> +       gdb_test_no_output "set sysroot ${sysroot}"
> +    }
> +
> +    gdb_load_shlib $::libobj
> +
> +    if ![runto_main] then {
> +       return 0
> +    }
> +
> +    # Get inferior's PID for later.
> +    set inferior_pid [get_inferior_pid]
> +
> +    # Run to the 'break-here' marker.
> +    gdb_breakpoint [gdb_get_line_number "break-here"]
> +    gdb_continue_to_breakpoint "break-here" ".* break-here .*"
> +
> +    set memfd ""
> +    gdb_test_multiple "p mem_fd" "Get file descriptor" {
> +       -re -wrap "\\\$$::decimal = (\[^\r\n\]*)" {
> +           set memfd $expect_out(1,string)
> +           pass $gdb_test_name
> +       }
> +    }
> +
> +    # The first call to open_library (in the test program) is done
> +    # with a canonical path.
> +    with_test_prefix "canonical path" {
> +       gdb_breakpoint "open_library"
> +       gdb_continue_to_breakpoint "open_library"
> +       validate_library_load $inferior_pid $memfd
> +    }
> +
> +    # The second call to open_library (in the test program) is done
> +    # with a non-canonical path, however, GDB should resolve this to a
> +    # canonical path for display to the user, so the output we see
> +    # should be unchanged.
> +    with_test_prefix "non-canonical path" {
> +       gdb_breakpoint "open_library"
> +       gdb_continue_to_breakpoint "open_library"
> +       validate_library_load $inferior_pid $memfd
> +    }
> +}
> +
> +# First run of the test.
> +do_test
> +
> +# Possible second run of the test.  If we are using a remote target then
> we
> +# should consider setting the sysroot to 'target:' and re-running the
> test.
> +if {[target_info exists gdb_protocol]
> +    && ([target_info gdb_protocol] == "remote"
> +       || [target_info gdb_protocol] == "extended-remote")} {
> +    # GDB will already be running after the first call to do_test, so we
> can
> +    # take a peek at the current sysroot setting, and decide if we should
> +    # repeat the test with a different setting.
> +
> +    set new_sysroot ""
> +    gdb_test_multiple "show sysroot" "" {
> +       -wrap -re "The current system root is \"\"\\." {
> +           pass $gdb_test_name
> +
> +           # Repeat the test with 'target:' sysroot.
> +           set new_sysroot "target:"
> +       }
> +       -wrap -re "The current system root is \"target:\"\\." {
> +           pass $gdb_test_name
> +
> +           # Nothing else to do, we already tested with target: sysroot.
> +       }
> +       -re "$gdb_prompt $" {
> +           pass $gdb_test_name
> +
> +           # If already testing with any other sysroot, we probably should
> +           # not try to adjust things, so don't do any further testing.
> +       }
> +    }
> +
> +    with_test_prefix "sysroot $new_sysroot" {
> +       if { $new_sysroot != "" } {
> +           do_test $new_sysroot
> +       }
> +    }
> +}
>
>