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Cache a copy of the user's shell on macOS

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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
2018-10-27  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.
This commit is contained in:
Tom Tromey
2018-09-17 12:29:22 -06:00
parent 36033ef57c
commit b50a8b9a91
2 changed files with 153 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
gdb/ChangeLog
View File

@@ -1,3 +1,12 @@
2018-10-27 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.
2018-10-27 Tom Tromey <tom@tromey.com>
* unittests/scoped_fd-selftests.c (test_to_file): New function.

151
gdb/darwin-nat.c
View File

@@ -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 ();
/* There's no need to insert a directory separator here, because
SHELL is known to be absolute. */
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);
}