[PATCH v2] Make sure GDB uses a valid shell when starting the inferior and to perform the "shell" command

Sergio Durigan Junior
2015-07-24T20:38:22Z

It is known that GDB needs a valid shell to start the inferior and to
offer the "shell" command to the user. This has recently been the
cause of a problem on the MIPS buildslave, because $SHELL was set to
/sbin/nologin and several tests were failing. The thread is here:

<https://sourceware.org/ml/gdb-patches/2015-07/msg00535.html>

However, I think we can do better than that. If 'startup-with-shell'
is on, which is the default, we blindly trust that the user will
provide a valid shell for us, and this may not be true all the time.
So I am proposing a patch to increment the tests made by GDB before
running the inferior to decide whether it will use $SHELL or not.
Particularly, I created a new function, called "valid_shell", which
defines the concept of a valid shell for GDB:

- A file that exists and is executable by the user

- A file that is not {,/usr}/sbin/nologin, nor /bin/false

For now, I think this is enough to cover most cases. The default
action when an invalid shell is found is to use /bin/sh instead (we
already do that when $SHELL is not defined, for example), and also
issue a warning to the user. This applies for when we are starting
the inferior and when we are executing the "shell" command.

To make things more robust, I made the code also check /bin/sh and
make sure it is also valid. If it is not, and if we are starting the
inferior, then GDB will use fork+exec instead. If we are executing
the "shell" command and we cannot find a valid shell, GDB will error
out.

I updated the documentation to reflect the new behavior, and created a
testcase to exercise the code. This patch has been regression-tested
on Fedora 22 x86_64.

OK to apply?

Changes from v1:

- Using @pxref instead of @ref in the documentation

- Don't run the testcase when the target is mingw, cygwin, or remote

- Including /usr/sbin/nologin and /bin/false in the list of invalid
shells

gdb/ChangeLog:
2015-07-24 Sergio Durigan Junior <sergiodj@redhat.com>

* cli/cli-cmds.c (shell_escape): Check if the selected shell is
valid.
* fork-child.c (check_startup_with_shell): New function.
(fork_inferior): Move code to the new function above. Call it.
* utils.c (valid_shell): New function.
* utils.h (valid_shell): New prototype.

gdb/doc/ChangeLog:
2015-07-24 Sergio Durigan Junior <sergiodj@redhat.com>

* gdb.texinfo (Shell Commands): Mention that the shell needs to be
valid; point to "Valid Shell" subsection.
(Valid Shell): New subsection.
(Starting your Program): Mention that the shell needs to be valid;
point to "Valid Shell" subsection.
(Your Program's Arguments): Likewise.
(Your Program's Environment): Likewise.

gdb/testsuite/ChangeLog:
2015-07-24 Sergio Durigan Junior <sergiodj@redhat.com>

* gdb.base/invalid-shell.exp: New file.
---
gdb/cli/cli-cmds.c | 9 +++-
gdb/doc/gdb.texinfo | 54 ++++++++++++++-------
gdb/fork-child.c | 82 +++++++++++++++++++++++++++-----
gdb/testsuite/gdb.base/invalid-shell.exp | 48 +++++++++++++++++++
gdb/utils.c | 12 +++++
gdb/utils.h | 13 +++++
6 files changed, 187 insertions(+), 31 deletions(-)
create mode 100644 gdb/testsuite/gdb.base/invalid-shell.exp

diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index 2ec2dd3..ed6a1df 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -752,8 +752,13 @@ shell_escape (char *arg, int from_tty)

close_most_fds ();

- if ((user_shell = (char *) getenv ("SHELL")) == NULL)
- user_shell = "/bin/sh";
+ if ((user_shell = (char *) getenv ("SHELL")) == NULL
+ || !valid_shell (user_shell))
+ {
+ user_shell = "/bin/sh";
+ if (!valid_shell (user_shell))
+ error (_("Cannot use '%s' as a shell."), user_shell);
+ }

/* Get the name of the shell for arg0. */
p = lbasename (user_shell);
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 9e2ecd1..ea1e71b 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -1409,11 +1409,12 @@ just use the @code{shell} command.
@cindex shell escape
@item shell @var{command-string}
@itemx !@var{command-string}
-Invoke a standard shell to execute @var{command-string}.
-Note that no space is needed between @code{!} and @var{command-string}.
-If it exists, the environment variable @code{SHELL} determines which
-shell to run. Otherwise @value{GDBN} uses the default shell
-(@file{/bin/sh} on Unix systems, @file{COMMAND.COM} on MS-DOS, etc.).
+Invoke a standard shell to execute @var{command-string}. Note that no
+space is needed between @code{!} and @var{command-string}. If it
+exists and points to a valid shell (@pxref{Valid Shell,,}), the
+environment variable @code{SHELL} determines which shell to run.
+Otherwise @value{GDBN} uses the default shell (@file{/bin/sh} on Unix
+systems, @file{COMMAND.COM} on MS-DOS, etc.).
@end table

The utility @code{make} is often needed in development environments.
@@ -1428,6 +1429,25 @@ Execute the @code{make} program with the specified
arguments. This is equivalent to @samp{shell make @var{make-args}}.
@end table

+@node Valid Shell
+@subsection Valid Shell
+
+@value{GDBN} considers a @emph{valid shell} a file that:
+
+@enumerate
+@item
+Exists and can be executed by the user.
+
+@item
+Is not the @file{/sbin/nologin} (or @file{/usr/sbin/nologin}) program.
+
+@item
+Is not the @file{/bin/false} program.
+@end enumerate
+
+If any of those conditions are not met, the specified shell is not
+used by @value{GDBN}.
+
@node Logging Output
@section Logging Output
@cindex logging @value{GDBN} output
@@ -2041,6 +2061,7 @@ is used to pass the arguments, so that you may use normal conventions
the arguments.
In Unix systems, you can control which shell is used with the
@code{SHELL} environment variable. If you do not define @code{SHELL},
+or if you define it to an invalid shell (@pxref{Valid Shell,,}),
@value{GDBN} uses the default shell (@file{/bin/sh}). You can disable
use of any shell with the @code{set startup-with-shell} command (see
below for details).
@@ -2286,9 +2307,10 @@ The arguments to your program can be specified by the arguments of the
@code{run} command.
They are passed to a shell, which expands wildcard characters and
performs redirection of I/O, and thence to your program. Your
-@code{SHELL} environment variable (if it exists) specifies what shell
-@value{GDBN} uses. If you do not define @code{SHELL}, @value{GDBN} uses
-the default shell (@file{/bin/sh} on Unix).
+@code{SHELL} environment variable (if it exists and if it contains a
+valid shell---@pxref{Valid Shell,,}) specifies what shell
+@value{GDBN} uses. If you do not define @code{SHELL}, @value{GDBN}
+uses the default shell (@file{/bin/sh} on Unix).

On non-Unix systems, the program is usually invoked directly by
@value{GDBN}, which emulates I/O redirection via the appropriate system
@@ -2395,14 +2417,14 @@ rather than assigning it an empty value.

@emph{Warning:} On Unix systems, @value{GDBN} runs your program using
the shell indicated by your @code{SHELL} environment variable if it
-exists (or @code{/bin/sh} if not). If your @code{SHELL} variable
-names a shell that runs an initialization file when started
-non-interactively---such as @file{.cshrc} for C-shell, $@file{.zshenv}
-for the Z shell, or the file specified in the @samp{BASH_ENV}
-environment variable for BASH---any variables you set in that file
-affect your program. You may wish to move setting of environment
-variables to files that are only run when you sign on, such as
-@file{.login} or @file{.profile}.
+exists and points to a valid shell (@pxref{Valid Shell,,}), or
+@code{/bin/sh} if not. If your @code{SHELL} variable names a shell
+that runs an initialization file when started non-interactively---such
+as @file{.cshrc} for C-shell, $@file{.zshenv} for the Z shell, or the
+file specified in the @samp{BASH_ENV} environment variable for
+BASH---any variables you set in that file affect your program. You
+may wish to move setting of environment variables to files that are
+only run when you sign on, such as @file{.login} or @file{.profile}.

@node Working Directory
@section Your Program's Working Directory
diff --git a/gdb/fork-child.c b/gdb/fork-child.c
index 4ba62b0..0e7e14c 100644
--- a/gdb/fork-child.c
+++ b/gdb/fork-child.c
@@ -108,6 +108,73 @@ escape_bang_in_quoted_argument (const char *shell_file)
return 0;
}

+/* Check if the user has specified any shell to start the inferior,
+ and if the specified shell is valid (i.e., it exists and can be
+ executed by the user).
+
+ The shell specified by the user, if any, is
+ SHELL_FILE_ARG.
+
+ DEFAULT_SHELL_FILE is the default shell that will be used by GDB if
+ the user said she wants to start the inferior using a shell (i.e.,
+ STARTUP_WITH_SHELL is 1), but:
+
+ - No shell has been specified by the user (i.e., the $SHELL
+ environment variable is NULL), or;
+
+ - The shell specified by the user (through the $SHELL environment
+ variable) is invalid. An invalid shell is defined as being
+ either a non-existing file, a file the user cannot execute, or
+ the /sbin/nologin shell.
+
+ This function will return 1 when the user wants to start the
+ inferior using a shell and the shell is valid; it will return 0
+ otherwise. */
+
+static int
+check_startup_with_shell (char **shell_file, char *shell_file_arg,
+ char *default_shell_file)
+{
+ /* 'startup_with_shell' is declared in inferior.h and bound to the
+ "set startup-with-shell" option. If 0, we'll just do a
+ fork/exec, no shell, so don't bother figuring out what shell. */
+ if (startup_with_shell)
+ {
+ gdb_assert (shell_file != NULL);
+ gdb_assert (default_shell_file != NULL);
+ *shell_file = shell_file_arg;
+
+ /* Figure out what shell to start up the user program under. */
+ if (*shell_file == NULL)
+ *shell_file = getenv ("SHELL");
+ if (*shell_file == NULL
+ || !valid_shell (*shell_file))
+ {
+ if (*shell_file != NULL)
+ warning (_("Invalid shell '%s'; using '%s' instead."),
+ *shell_file, default_shell_file);
+ /* Being overzealous here. */
+ if (valid_shell (default_shell_file))
+ *shell_file = default_shell_file;
+ else
+ {
+ warning (_("Cannot use '%s' as the shell, using 'exec' "
+ "to start inferior."),
+ default_shell_file);
+ *shell_file = NULL;
+ }
+ }
+
+ if (*shell_file != NULL)
+ return 1;
+ }
+
+ /* We set startup_with_shell to zero here because if the specified
+ shell is invalid then no shell will be used. */
+ startup_with_shell = 0;
+ return 0;
+}
+
/* Start an inferior Unix child process and sets inferior_ptid to its
pid. EXEC_FILE is the file to run. ALLARGS is a string containing
the arguments to the program. ENV is the environment vector to
@@ -148,19 +215,8 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
if (exec_file == 0)
exec_file = get_exec_file (1);

- /* 'startup_with_shell' is declared in inferior.h and bound to the
- "set startup-with-shell" option. If 0, we'll just do a
- fork/exec, no shell, so don't bother figuring out what shell. */
- shell_file = shell_file_arg;
- if (startup_with_shell)
- {
- /* Figure out what shell to start up the user program under. */
- if (shell_file == NULL)
- shell_file = getenv ("SHELL");
- if (shell_file == NULL)
- shell_file = default_shell_file;
- shell = 1;
- }
+ shell = check_startup_with_shell (&shell_file, shell_file_arg,
+ default_shell_file);

if (!shell)
{
diff --git a/gdb/testsuite/gdb.base/invalid-shell.exp b/gdb/testsuite/gdb.base/invalid-shell.exp
new file mode 100644
index 0000000..6593ba6
--- /dev/null
+++ b/gdb/testsuite/gdb.base/invalid-shell.exp
@@ -0,0 +1,48 @@
+# Copyright 2015 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/>.
+
+standard_testfile normal.c
+
+if { [is_remote target] || ![isnative] } {
+ untested "cannot test on remote targets"
+ return -1
+}
+
+if { [istarget "*-*-mingw*"] || [istarget "*-*-cygwin*"] } {
+ untested "cannot test on mingw or cygwin"
+ return -1
+}
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile debug] } {
+ untested "could not compile test program"
+ return -1
+}
+
+gdb_exit
+
+# Set the $SHELL to an invalid file. This will cause GDB to use
+# /bin/sh instead.
+set oldshell $env(SHELL)
+set env(SHELL) "/invalid/path/to/file"
+
+clean_restart $binfile
+
+# Running the inferior must work.
+gdb_test "run" "Starting program: .*\r\nwarning: Invalid shell \\\'/invalid/path/to/file\\\'; using \\\'/bin/sh\\\' instead.\r\n\\\[Inferior $decimal \\\(process $decimal\\\) exited normally\\\]" "starting with /bin/sh instead of invalid shell"
+
+# Invoking a shell command must also work.
+gdb_test "shell echo hi" "hi" "invoking shell command from prompt"
+
+set env(SHELL) $oldshell
diff --git a/gdb/utils.c b/gdb/utils.c
index acb4c7d..11c1134 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -3364,6 +3364,18 @@ gdb_filename_fnmatch (const char *pattern, const char *string, int flags)
return fnmatch (pattern, string, flags);
}

+/* See utils.h. */
+
+int
+valid_shell (const char *shell)
+{
+ return (shell != NULL
+ && strcmp (shell, "/sbin/nologin") != 0
+ && strcmp (shell, "/usr/sbin/nologin") != 0
+ && strcmp (shell, "/bin/false") != 0
+ && access (shell, X_OK) == 0);
+}
+
/* Provide a prototype to silence -Wmissing-prototypes. */
extern initialize_file_ftype _initialize_utils;

diff --git a/gdb/utils.h b/gdb/utils.h
index 995a1cf..2657708 100644
--- a/gdb/utils.h
+++ b/gdb/utils.h
@@ -369,4 +369,17 @@ extern void dump_core (void);

extern char *make_hex_string (const gdb_byte *data, size_t length);

+/* Return 1 if SHELL is a valid shell to execute a command, zero
+ otherwise.
+
+ A valid shell is defined a file that is:
+
+ - Not {,/usr}/sbin/nologin;
+
+ - Not /bin/false;
+
+ - Executable by the user. */
+
+extern int valid_shell (const char *shell);
+
#endif /* UTILS_H */
--
2.4.3