[v2,6/6] Cache a copy of the user's shell on macOS

Message ID 20181018223100.20693-7-tom@tromey.com
State New
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Series
  • A different approach to startup-with-shell on macOS
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Commit Message

Tom Tromey Oct. 18, 2018, 10:31 p.m.
Recent versions of macOS have a feature called System Integrity
Protection.  Among other things, This feature prevents ptrace from
tracing certain programs --- for example, the programs in /bin, which
includes typical shells.

This means that startup-with-shell does not work properly.  This is PR
cli/23364.  Currently there is a workaround in gdb to disable
startup-with-shell when this feature might be in use.

This patch changes gdb to be a bit more precise about when
startup-with-shell will not work, by checking whether the shell
executable is restricted.

If the shell is restricted, then this patch will also cause gdb to
cache a copy of the shell in the gdb cache directory, and then reset
the SHELL environment variable to point to this copy.  This lets
startup-with-shell work again.

Tested on High Sierra by trying to start a program using redirection,
and by running startup-with-shell.exp.

gdb/ChangeLog
2018-10-18  Tom Tromey  <tom@tromey.com>

	PR cli/23364:
	* darwin-nat.c (copied_shell): New global.
	(may_have_sip): Rename from should_disable_startup_with_shell.
	(copy_shell_to_cache, maybe_cache_shell): New functions.
	(darwin_nat_target::create_inferior): Update.  Use
	copied_shell.
---
 gdb/ChangeLog    |   9 +++
 gdb/darwin-nat.c | 151 ++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 153 insertions(+), 7 deletions(-)

-- 
2.17.1

Comments

Simon Marchi Oct. 20, 2018, 2:35 a.m. | #1
On 2018-10-18 6:31 p.m., Tom Tromey wrote:
> +/* If $SHELL is restricted, try to cache a copy.  Starting with El

> +   Capitan, macOS introduced System Integrity Protection.  Among other

> +   things, this prevents certain executables from being ptrace'd.  In

> +   particular, executables in /bin, like most shells, are affected.

> +   To work around this, while preserving command-line glob expansion

> +   and redirections, gdb will cache a copy of the shell.  Return true

> +   if all is well -- either the shell is not subject to SIP or it has

> +   been successfully cached.  Returns false if something failed.  */

> +

> +static bool

> +maybe_cache_shell ()

> +{

> +  /* SF_RESTRICTED is defined in sys/stat.h and lets us determine if a

> +     given file is subject to SIP.  */

> +#ifdef SF_RESTRICTED

> +

> +  /* If a check fails we want to revert -- maybe the user deleted the

> +     cache while gdb was running, or something like that.  */

> +  copied_shell = nullptr;

> +

> +  const char *shell = get_shell ();

> +  if (!IS_ABSOLUTE_PATH (shell))

> +    {

> +      warning (_("This version of macOS has System Integrity Protection.\n\

> +Normally gdb would try to work around this by caching a copy of your shell,\n\

> +but because your shell (%s) is not an absolute path, this is being skipped."),

> +	       shell);

> +      return false;

> +    }

> +

> +  struct stat sb;

> +  if (stat (shell, &sb) < 0)

> +    {

> +      warning (_("This version of macOS has System Integrity Protection.\n\

> +Normally gdb would try to work around this by caching a copy of your shell,\n\

> +but because gdb could not stat your shell (%s), this is being skipped.\n\

> +The error was: %s"),

> +	       shell, safe_strerror (errno));

> +      return false;

> +    }

> +

> +  if ((sb.st_flags & SF_RESTRICTED) == 0)

> +    return true;

> +

> +  /* Put the copy somewhere like ~/Library/Caches/gdb/bin/sh.  */

> +  std::string new_name = get_standard_cache_dir ();

> +  if (!IS_DIR_SEPARATOR (new_name.back ()) && !IS_ABSOLUTE_PATH (shell))


I mentioned something about this on the version of the patch, but you might
have missed it since you didn't reply to that specifically, so I'll send
it again just to be sure:

I believe this !IS_ABSOLUTE_PATH check can never be true, since we would
have returned early if it was the case?  If so, this append is not needed

Simon
Tom Tromey Oct. 27, 2018, 5:42 p.m. | #2
>>>>> "Simon" == Simon Marchi <simark@simark.ca> writes:


>> +  /* Put the copy somewhere like ~/Library/Caches/gdb/bin/sh.  */

>> +  std::string new_name = get_standard_cache_dir ();

>> +  if (!IS_DIR_SEPARATOR (new_name.back ()) && !IS_ABSOLUTE_PATH (shell))


Simon> I mentioned something about this on the version of the patch, but you might
Simon> have missed it since you didn't reply to that specifically, so I'll send
Simon> it again just to be sure:

Simon> I believe this !IS_ABSOLUTE_PATH check can never be true, since we would
Simon> have returned early if it was the case?  If so, this append is not needed

I will remove the redundant check.
I think I changed this at some point and forgot to update that.

Tom

Patch

diff --git a/gdb/darwin-nat.c b/gdb/darwin-nat.c
index ecc7635d04..1c6f8e061b 100644
--- a/gdb/darwin-nat.c
+++ b/gdb/darwin-nat.c
@@ -38,6 +38,7 @@ 
 #include "bfd.h"
 #include "bfd/mach-o.h"
 
+#include <copyfile.h>
 #include <sys/ptrace.h>
 #include <sys/signal.h>
 #include <setjmp.h>
@@ -61,7 +62,11 @@ 
 #include <mach/port.h>
 
 #include "darwin-nat.h"
+#include "filenames.h"
 #include "common/filestuff.h"
+#include "common/gdb_unlinker.h"
+#include "common/pathstuff.h"
+#include "common/scoped_fd.h"
 #include "nat/fork-inferior.h"
 
 /* Quick overview.
@@ -119,6 +124,10 @@  static int enable_mach_exceptions;
 /* Inferior that should report a fake stop event.  */
 static struct inferior *darwin_inf_fake_stop;
 
+/* If non-NULL, the shell we actually invoke.  See maybe_cache_shell
+   for details.  */
+static const char *copied_shell;
+
 #define PAGE_TRUNC(x) ((x) & ~(mach_page_size - 1))
 #define PAGE_ROUND(x) PAGE_TRUNC((x) + mach_page_size - 1)
 
@@ -1833,10 +1842,11 @@  darwin_execvp (const char *file, char * const argv[], char * const env[])
   posix_spawnp (NULL, argv[0], NULL, &attr, argv, env);
 }
 
-/* Read kernel version, and return TRUE on Sierra or later.  */
+/* Read kernel version, and return TRUE if this host may have System
+   Integrity Protection (Sierra or later).  */
 
 static bool
-should_disable_startup_with_shell ()
+may_have_sip ()
 {
   char str[16];
   size_t sz = sizeof (str);
@@ -1852,6 +1862,131 @@  should_disable_startup_with_shell ()
   return false;
 }
 
+/* A helper for maybe_cache_shell.  This copies the shell to the
+   cache.  It will throw an exception on any failure.  */
+
+static void
+copy_shell_to_cache (const char *shell, const std::string &new_name)
+{
+  scoped_fd from_fd (gdb_open_cloexec (shell, O_RDONLY, 0));
+  if (from_fd.get () < 0)
+    error (_("Could not open shell (%s) for reading: %s"),
+	   shell, safe_strerror (errno));
+
+  std::string new_dir = ldirname (new_name.c_str ());
+  if (!mkdir_recursive (new_dir.c_str ()))
+    error (_("Could not make cache directory \"%s\": %s"),
+	   new_dir.c_str (), safe_strerror (errno));
+
+  gdb::char_vector temp_name = make_temp_filename (new_name);
+  scoped_fd to_fd (gdb_mkostemp_cloexec (&temp_name[0]));
+  gdb::unlinker unlink_file_on_error (temp_name.data ());
+
+  if (to_fd.get () < 0)
+    error (_("Could not open temporary file \"%s\" for writing: %s"),
+	   temp_name.data (), safe_strerror (errno));
+
+  if (fcopyfile (from_fd.get (), to_fd.get (), nullptr,
+		 COPYFILE_STAT | COPYFILE_DATA) != 0)
+    error (_("Could not copy shell to cache as \"%s\": %s"),
+	   temp_name.data (), safe_strerror (errno));
+
+  /* Be sure that the caching is atomic so that we don't get bad
+     results from multiple copies of gdb running at the same time.  */
+  if (rename (temp_name.data (), new_name.c_str ()) != 0)
+    error (_("Could not rename shell cache file to \"%s\": %s"),
+	   new_name.c_str (), safe_strerror (errno));
+
+  unlink_file_on_error.keep ();
+}
+
+/* If $SHELL is restricted, try to cache a copy.  Starting with El
+   Capitan, macOS introduced System Integrity Protection.  Among other
+   things, this prevents certain executables from being ptrace'd.  In
+   particular, executables in /bin, like most shells, are affected.
+   To work around this, while preserving command-line glob expansion
+   and redirections, gdb will cache a copy of the shell.  Return true
+   if all is well -- either the shell is not subject to SIP or it has
+   been successfully cached.  Returns false if something failed.  */
+
+static bool
+maybe_cache_shell ()
+{
+  /* SF_RESTRICTED is defined in sys/stat.h and lets us determine if a
+     given file is subject to SIP.  */
+#ifdef SF_RESTRICTED
+
+  /* If a check fails we want to revert -- maybe the user deleted the
+     cache while gdb was running, or something like that.  */
+  copied_shell = nullptr;
+
+  const char *shell = get_shell ();
+  if (!IS_ABSOLUTE_PATH (shell))
+    {
+      warning (_("This version of macOS has System Integrity Protection.\n\
+Normally gdb would try to work around this by caching a copy of your shell,\n\
+but because your shell (%s) is not an absolute path, this is being skipped."),
+	       shell);
+      return false;
+    }
+
+  struct stat sb;
+  if (stat (shell, &sb) < 0)
+    {
+      warning (_("This version of macOS has System Integrity Protection.\n\
+Normally gdb would try to work around this by caching a copy of your shell,\n\
+but because gdb could not stat your shell (%s), this is being skipped.\n\
+The error was: %s"),
+	       shell, safe_strerror (errno));
+      return false;
+    }
+
+  if ((sb.st_flags & SF_RESTRICTED) == 0)
+    return true;
+
+  /* Put the copy somewhere like ~/Library/Caches/gdb/bin/sh.  */
+  std::string new_name = get_standard_cache_dir ();
+  if (!IS_DIR_SEPARATOR (new_name.back ()) && !IS_ABSOLUTE_PATH (shell))
+    new_name.push_back ('/');
+  new_name.append (shell);
+
+  /* Maybe it was cached by some earlier gdb.  */
+  if (stat (new_name.c_str (), &sb) != 0 || !S_ISREG (sb.st_mode))
+    {
+      TRY
+	{
+	  copy_shell_to_cache (shell, new_name);
+	}
+      CATCH (ex, RETURN_MASK_ERROR)
+	{
+	  warning (_("This version of macOS has System Integrity Protection.\n\
+Because `startup-with-shell' is enabled, gdb tried to work around SIP by\n\
+caching a copy of your shell.  However, this failed:\n\
+%s\n\
+If you correct the problem, gdb will automatically try again the next time\n\
+you \"run\".  To prevent these attempts, you can use:\n\
+    set startup-with-shell off"),
+		   ex.message);
+	  return false;
+	}
+      END_CATCH
+
+      printf_filtered (_("Note: this version of macOS has System Integrity Protection.\n\
+Because `startup-with-shell' is enabled, gdb has worked around this by\n\
+caching a copy of your shell.  The shell used by \"run\" is now:\n\
+    %s\n"),
+		       new_name.c_str ());
+    }
+
+  /* We need to make sure that the new name has the correct lifetime.  */
+  static std::string saved_shell = std::move (new_name);
+  copied_shell = saved_shell.c_str ();
+
+#endif /* SF_RESTRICTED */
+
+  return true;
+}
+
 void
 darwin_nat_target::create_inferior (const char *exec_file,
 				    const std::string &allargs,
@@ -1859,16 +1994,18 @@  darwin_nat_target::create_inferior (const char *exec_file,
 {
   gdb::optional<scoped_restore_tmpl<int>> restore_startup_with_shell;
 
-  if (startup_with_shell && should_disable_startup_with_shell ())
+  if (startup_with_shell && may_have_sip ())
     {
-      warning (_("startup-with-shell not supported on this macOS version,"
-	         " disabling it."));
-      restore_startup_with_shell.emplace (&startup_with_shell, 0);
+      if (!maybe_cache_shell ())
+	{
+	  warning (_("startup-with-shell is now temporarily disabled"));
+	  restore_startup_with_shell.emplace (&startup_with_shell, 0);
+	}
     }
 
   /* Do the hard work.  */
   fork_inferior (exec_file, allargs, env, darwin_ptrace_me,
-		 darwin_ptrace_him, darwin_pre_ptrace, NULL,
+		 darwin_ptrace_him, darwin_pre_ptrace, copied_shell,
 		 darwin_execvp);
 }