[v3,2/2] nss: Protect against errno changes in function lookup (bug 28953)

  dlopen may clobber errno.  The nss_test_errno module uses an ELF
constructor to achieve that, but there could be internal errors
during dlopen that cause this, too.  Therefore, the NSS framework
has to guard against such errno clobbers.

__nss_module_get_function is currently the only function that calls
__nss_module_load, so it is sufficient to save and restore errno
around this call.
---
v3: Do not change errno on module load failure due to existing ambiguity.

 nss/Makefile             | 15 ++++++++--
 nss/nss_module.c         | 12 +++++++-
 nss/nss_test_errno.c     | 58 ++++++++++++++++++++++++++++++++++++++
 nss/tst-nss-test_errno.c | 61 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 142 insertions(+), 4 deletions(-)
 create mode 100644 nss/nss_test_errno.c
 create mode 100644 nss/tst-nss-test_errno.c
  

On Thu, Mar 10, 2022 at 1:19 PM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> dlopen may clobber errno.  The nss_test_errno module uses an ELF
> constructor to achieve that, but there could be internal errors
> during dlopen that cause this, too.  Therefore, the NSS framework
> has to guard against such errno clobbers.

elf constructors may also clobber errno.. I know the standard does not
require errno to be sane on entering main() but it would be awesome if
that was always the case..or that the real constraints and pitfalls of
them are documented on the gcc manual page, that is currently very
vague about it..
Other than saving and restoring errno..what else ? do they have to be
as-safe right ?  this is not mentioned anywhere...or Im blind,

>
> __nss_module_get_function is currently the only function that calls
> __nss_module_load, so it is sufficient to save and restore errno
> around this call.
> ---
> v3: Do not change errno on module load failure due to existing ambiguity.
>
>  nss/Makefile             | 15 ++++++++--
>  nss/nss_module.c         | 12 +++++++-
>  nss/nss_test_errno.c     | 58 ++++++++++++++++++++++++++++++++++++++
>  nss/tst-nss-test_errno.c | 61 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 142 insertions(+), 4 deletions(-)
>  create mode 100644 nss/nss_test_errno.c
>  create mode 100644 nss/tst-nss-test_errno.c
>
> diff --git a/nss/Makefile b/nss/Makefile
> index 74e2c2426c..de439d4911 100644
> --- a/nss/Makefile
> +++ b/nss/Makefile
> @@ -60,7 +60,8 @@ tests                 = test-netdb test-digits-dots tst-nss-getpwent bug17079 \
>                           tst-nss-test1 \
>                           tst-nss-test2 \
>                           tst-nss-test4 \
> -                         tst-nss-test5
> +                         tst-nss-test5 \
> +                         tst-nss-test_errno
>  xtests                 = bug-erange
>
>  tests-container = \
> @@ -132,7 +133,7 @@ libnss_compat-inhibit-o     = $(filter-out .os,$(object-suffixes))
>  ifeq ($(build-static-nss),yes)
>  tests-static           += tst-nss-static
>  endif
> -extra-test-objs                += nss_test1.os nss_test2.os
> +extra-test-objs                += nss_test1.os nss_test2.os nss_test_errno.os
>
>  include ../Rules
>
> @@ -166,19 +167,26 @@ rtld-tests-LDFLAGS += -Wl,--dynamic-list=nss_test.ver
>
>  libof-nss_test1 = extramodules
>  libof-nss_test2 = extramodules
> +libof-nss_test_errno = extramodules
>  $(objpfx)/libnss_test1.so: $(objpfx)nss_test1.os $(link-libc-deps)
>         $(build-module)
>  $(objpfx)/libnss_test2.so: $(objpfx)nss_test2.os $(link-libc-deps)
>         $(build-module)
> +$(objpfx)/libnss_test_errno.so: $(objpfx)nss_test_errno.os $(link-libc-deps)
> +       $(build-module)
>  $(objpfx)nss_test2.os : nss_test1.c
>  # Use the nss_files suffix for these objects as well.
>  $(objpfx)/libnss_test1.so$(libnss_files.so-version): $(objpfx)/libnss_test1.so
>         $(make-link)
>  $(objpfx)/libnss_test2.so$(libnss_files.so-version): $(objpfx)/libnss_test2.so
>         $(make-link)
> +$(objpfx)/libnss_test_errno.so$(libnss_files.so-version): \
> +  $(objpfx)/libnss_test_errno.so
> +       $(make-link)
>  $(patsubst %,$(objpfx)%.out,$(tests) $(tests-container)) : \
>         $(objpfx)/libnss_test1.so$(libnss_files.so-version) \
> -       $(objpfx)/libnss_test2.so$(libnss_files.so-version)
> +       $(objpfx)/libnss_test2.so$(libnss_files.so-version) \
> +       $(objpfx)/libnss_test_errno.so$(libnss_files.so-version)
>
>  ifeq (yes,$(have-thread-library))
>  $(objpfx)tst-cancel-getpwuid_r: $(shared-thread-library)
> @@ -194,3 +202,4 @@ LDFLAGS-tst-nss-test2 = -Wl,--disable-new-dtags
>  LDFLAGS-tst-nss-test3 = -Wl,--disable-new-dtags
>  LDFLAGS-tst-nss-test4 = -Wl,--disable-new-dtags
>  LDFLAGS-tst-nss-test5 = -Wl,--disable-new-dtags
> +LDFLAGS-tst-nss-test_errno = -Wl,--disable-new-dtags
> diff --git a/nss/nss_module.c b/nss/nss_module.c
> index f9a1263e5a..f00bbd9e1a 100644
> --- a/nss/nss_module.c
> +++ b/nss/nss_module.c
> @@ -330,8 +330,18 @@ name_search (const void *left, const void *right)
>  void *
>  __nss_module_get_function (struct nss_module *module, const char *name)
>  {
> +  /* A successful dlopen might clobber errno.   */
> +  int saved_errno = errno;
> +
>    if (!__nss_module_load (module))
> -    return NULL;
> +    {
> +      /* Reporting module load failure is currently inaccurate.  See
> +        bug 22041.  Not changing errno is the conservative choice.  */
> +      __set_errno (saved_errno);
> +      return NULL;
> +    }
> +
> +  __set_errno (saved_errno);
>
>    function_name *name_entry = bsearch (name, nss_function_name_array,
>                                         array_length (nss_function_name_array),
> diff --git a/nss/nss_test_errno.c b/nss/nss_test_errno.c
> new file mode 100644
> index 0000000000..680f8a07b9
> --- /dev/null
> +++ b/nss/nss_test_errno.c
> @@ -0,0 +1,58 @@
> +/* NSS service provider with errno clobber.
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library 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
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <errno.h>
> +#include <nss.h>
> +#include <stdlib.h>
> +
> +/* Catch misnamed and functions.  */
> +#pragma GCC diagnostic error "-Wmissing-prototypes"
> +NSS_DECLARE_MODULE_FUNCTIONS (test_errno)
> +
> +static void __attribute__ ((constructor))
> +init (void)
> +{
> +  /* An arbitrary error code which is otherwise not used.  */
> +  errno = ELIBBAD;
> +}
> +
> +/* Lookup functions for pwd follow that do not return any data.  */
> +
> +/* Catch misnamed function definitions.  */
> +
> +enum nss_status
> +_nss_test_errno_setpwent (int stayopen)
> +{
> +  setenv ("_nss_test_errno_setpwent", "yes", 1);
> +  return NSS_STATUS_SUCCESS;
> +}
> +
> +enum nss_status
> +_nss_test_errno_getpwent_r (struct passwd *result,
> +                            char *buffer, size_t size, int *errnop)
> +{
> +  setenv ("_nss_test_errno_getpwent_r", "yes", 1);
> +  return NSS_STATUS_NOTFOUND;
> +}
> +
> +enum nss_status
> +_nss_test_errno_endpwent (void)
> +{
> +  setenv ("_nss_test_errno_endpwent", "yes", 1);
> +  return NSS_STATUS_SUCCESS;
> +}
> diff --git a/nss/tst-nss-test_errno.c b/nss/tst-nss-test_errno.c
> new file mode 100644
> index 0000000000..d2c42dd363
> --- /dev/null
> +++ b/nss/tst-nss-test_errno.c
> @@ -0,0 +1,61 @@
> +/* getpwent failure when dlopen clobbers errno (bug 28953).
> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library 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
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <nss.h>
> +#include <support/check.h>
> +#include <stdlib.h>
> +#include <errno.h>
> +#include <stdbool.h>
> +#include <pwd.h>
> +#include <string.h>
> +
> +static int
> +do_test (void)
> +{
> +  __nss_configure_lookup ("passwd", "files test_errno");
> +
> +  errno = 0;
> +  setpwent ();
> +  TEST_COMPARE (errno, 0);
> +
> +  bool root_seen = false;
> +  while (true)
> +    {
> +      errno = 0;
> +      struct passwd *e = getpwent ();
> +      if (e == NULL)
> +        break;
> +      if (strcmp (e->pw_name, "root"))
> +        root_seen = true;
> +    }
> +
> +  TEST_COMPARE (errno, 0);
> +  TEST_VERIFY (root_seen);
> +
> +  errno = 0;
> +  endpwent ();
> +  TEST_COMPARE (errno, 0);
> +
> +  TEST_COMPARE_STRING (getenv ("_nss_test_errno_setpwent"), "yes");
> +  TEST_COMPARE_STRING (getenv ("_nss_test_errno_getpwent_r"), "yes");
> +  TEST_COMPARE_STRING (getenv ("_nss_test_errno_endpwent"), "yes");
> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>
> --
> 2.35.1
  

On 3/10/22 11:18, Florian Weimer via Libc-alpha wrote:
> dlopen may clobber errno.  The nss_test_errno module uses an ELF
> constructor to achieve that, but there could be internal errors
> during dlopen that cause this, too.  Therefore, the NSS framework
> has to guard against such errno clobbers.

LGTM.

Addresses Andreas' other comments about the the failed return, and the missing
license etc.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
 
> __nss_module_get_function is currently the only function that calls
> __nss_module_load, so it is sufficient to save and restore errno
> around this call.

Agreed.

> ---
> v3: Do not change errno on module load failure due to existing ambiguity.
> 
>  nss/Makefile             | 15 ++++++++--
>  nss/nss_module.c         | 12 +++++++-
>  nss/nss_test_errno.c     | 58 ++++++++++++++++++++++++++++++++++++++
>  nss/tst-nss-test_errno.c | 61 ++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 142 insertions(+), 4 deletions(-)
>  create mode 100644 nss/nss_test_errno.c
>  create mode 100644 nss/tst-nss-test_errno.c
> 
> diff --git a/nss/Makefile b/nss/Makefile
> index 74e2c2426c..de439d4911 100644
> --- a/nss/Makefile
> +++ b/nss/Makefile
> @@ -60,7 +60,8 @@ tests			= test-netdb test-digits-dots tst-nss-getpwent bug17079 \
>  			  tst-nss-test1 \
>  			  tst-nss-test2 \
>  			  tst-nss-test4 \
> -			  tst-nss-test5
> +			  tst-nss-test5 \
> +			  tst-nss-test_errno

OK.

>  xtests			= bug-erange
>  
>  tests-container = \
> @@ -132,7 +133,7 @@ libnss_compat-inhibit-o	= $(filter-out .os,$(object-suffixes))
>  ifeq ($(build-static-nss),yes)
>  tests-static		+= tst-nss-static
>  endif
> -extra-test-objs		+= nss_test1.os nss_test2.os
> +extra-test-objs		+= nss_test1.os nss_test2.os nss_test_errno.os

OK.

>  
>  include ../Rules
>  
> @@ -166,19 +167,26 @@ rtld-tests-LDFLAGS += -Wl,--dynamic-list=nss_test.ver
>  
>  libof-nss_test1 = extramodules
>  libof-nss_test2 = extramodules
> +libof-nss_test_errno = extramodules

OK.

>  $(objpfx)/libnss_test1.so: $(objpfx)nss_test1.os $(link-libc-deps)
>  	$(build-module)
>  $(objpfx)/libnss_test2.so: $(objpfx)nss_test2.os $(link-libc-deps)
>  	$(build-module)
> +$(objpfx)/libnss_test_errno.so: $(objpfx)nss_test_errno.os $(link-libc-deps)
> +	$(build-module)

OK.

>  $(objpfx)nss_test2.os : nss_test1.c
>  # Use the nss_files suffix for these objects as well.
>  $(objpfx)/libnss_test1.so$(libnss_files.so-version): $(objpfx)/libnss_test1.so
>  	$(make-link)
>  $(objpfx)/libnss_test2.so$(libnss_files.so-version): $(objpfx)/libnss_test2.so
>  	$(make-link)
> +$(objpfx)/libnss_test_errno.so$(libnss_files.so-version): \
> +  $(objpfx)/libnss_test_errno.so
> +	$(make-link)

OK.

>  $(patsubst %,$(objpfx)%.out,$(tests) $(tests-container)) : \
>  	$(objpfx)/libnss_test1.so$(libnss_files.so-version) \
> -	$(objpfx)/libnss_test2.so$(libnss_files.so-version)
> +	$(objpfx)/libnss_test2.so$(libnss_files.so-version) \
> +	$(objpfx)/libnss_test_errno.so$(libnss_files.so-version)

OK.

>  
>  ifeq (yes,$(have-thread-library))
>  $(objpfx)tst-cancel-getpwuid_r: $(shared-thread-library)
> @@ -194,3 +202,4 @@ LDFLAGS-tst-nss-test2 = -Wl,--disable-new-dtags
>  LDFLAGS-tst-nss-test3 = -Wl,--disable-new-dtags
>  LDFLAGS-tst-nss-test4 = -Wl,--disable-new-dtags
>  LDFLAGS-tst-nss-test5 = -Wl,--disable-new-dtags
> +LDFLAGS-tst-nss-test_errno = -Wl,--disable-new-dtags

OK.

> diff --git a/nss/nss_module.c b/nss/nss_module.c
> index f9a1263e5a..f00bbd9e1a 100644
> --- a/nss/nss_module.c
> +++ b/nss/nss_module.c
> @@ -330,8 +330,18 @@ name_search (const void *left, const void *right)
>  void *
>  __nss_module_get_function (struct nss_module *module, const char *name)
>  {
> +  /* A successful dlopen might clobber errno.   */
> +  int saved_errno = errno;

OK. Save on entry.

> +
>    if (!__nss_module_load (module))
> -    return NULL;
> +    {
> +      /* Reporting module load failure is currently inaccurate.  See
> +	 bug 22041.  Not changing errno is the conservative choice.  */
> +      __set_errno (saved_errno);
> +      return NULL;

OK. Agreed.

> +    }
> +
> +  __set_errno (saved_errno);

OK. Nothing after this would set errno.

>  
>    function_name *name_entry = bsearch (name, nss_function_name_array,
>                                         array_length (nss_function_name_array),
> diff --git a/nss/nss_test_errno.c b/nss/nss_test_errno.c
> new file mode 100644
> index 0000000000..680f8a07b9
> --- /dev/null
> +++ b/nss/nss_test_errno.c
> @@ -0,0 +1,58 @@
> +/* NSS service provider with errno clobber.

OK.

> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library 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
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <errno.h>
> +#include <nss.h>
> +#include <stdlib.h>
> +
> +/* Catch misnamed and functions.  */
> +#pragma GCC diagnostic error "-Wmissing-prototypes"

OK. Belt-and-suspenders.

> +NSS_DECLARE_MODULE_FUNCTIONS (test_errno)
> +
> +static void __attribute__ ((constructor))
> +init (void)
> +{
> +  /* An arbitrary error code which is otherwise not used.  */
> +  errno = ELIBBAD;
> +}
> +
> +/* Lookup functions for pwd follow that do not return any data.  */
> +
> +/* Catch misnamed function definitions.  */
> +
> +enum nss_status
> +_nss_test_errno_setpwent (int stayopen)
> +{
> +  setenv ("_nss_test_errno_setpwent", "yes", 1);
> +  return NSS_STATUS_SUCCESS;
> +}
> +
> +enum nss_status
> +_nss_test_errno_getpwent_r (struct passwd *result,
> +                            char *buffer, size_t size, int *errnop)
> +{
> +  setenv ("_nss_test_errno_getpwent_r", "yes", 1);
> +  return NSS_STATUS_NOTFOUND;
> +}
> +
> +enum nss_status
> +_nss_test_errno_endpwent (void)
> +{
> +  setenv ("_nss_test_errno_endpwent", "yes", 1);
> +  return NSS_STATUS_SUCCESS;
> +}
> diff --git a/nss/tst-nss-test_errno.c b/nss/tst-nss-test_errno.c
> new file mode 100644
> index 0000000000..d2c42dd363
> --- /dev/null
> +++ b/nss/tst-nss-test_errno.c
> @@ -0,0 +1,61 @@
> +/* getpwent failure when dlopen clobbers errno (bug 28953).

OK.

> +   Copyright (C) 2022 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library 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
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <nss.h>
> +#include <support/check.h>
> +#include <stdlib.h>
> +#include <errno.h>
> +#include <stdbool.h>
> +#include <pwd.h>
> +#include <string.h>
> +
> +static int
> +do_test (void)
> +{
> +  __nss_configure_lookup ("passwd", "files test_errno");

OK. Use test NSS provider.

> +
> +  errno = 0;
> +  setpwent ();

OK. Constructor changes things.

> +  TEST_COMPARE (errno, 0);
> +
> +  bool root_seen = false;
> +  while (true)
> +    {
> +      errno = 0;
> +      struct passwd *e = getpwent ();
> +      if (e == NULL)
> +        break;
> +      if (strcmp (e->pw_name, "root"))
> +        root_seen = true;
> +    }
> +
> +  TEST_COMPARE (errno, 0);

OK.

> +  TEST_VERIFY (root_seen);
> +
> +  errno = 0;
> +  endpwent ();

OK.

> +  TEST_COMPARE (errno, 0);
> +
> +  TEST_COMPARE_STRING (getenv ("_nss_test_errno_setpwent"), "yes");
> +  TEST_COMPARE_STRING (getenv ("_nss_test_errno_getpwent_r"), "yes");
> +  TEST_COMPARE_STRING (getenv ("_nss_test_errno_endpwent"), "yes");

OK. Slightly unorthodox to use an environment variable, but simple and useful.

> +
> +  return 0;
> +}
> +
> +#include <support/test-driver.c>