[PATCH] landlock: Work around randstruct unnamed static initializer support

[PATCH] landlock: Work around randstruct unnamed static initializer support

[Kees Cook]

Unnamed static initializers aren't supported by the randstruct GCC
plugin. Quoting the plugin, "set up a bogus anonymous struct field
designed to error out on unnamed struct initializers as gcc provides
no other way to detect such code". That is exactly what happens
with the landlock code, so adjust the static initializers for structs
lsm_ioctlop_audit and landlock_request that contain a randomized structure
(struct path) to use named variables, which avoids the intentional
GCC crashes:

security/landlock/fs.c: In function 'hook_file_ioctl_common':
security/landlock/fs.c:1745:61: internal compiler error: in count_type_elements, at expr.cc:7092
 1745 |                         .u.op = &(struct lsm_ioctlop_audit) {
      |                                                             ^

security/landlock/fs.c: In function 'log_fs_change_topology_path':
security/landlock/fs.c:1379:65: internal compiler error: in count_type_elements, at expr.cc:7092
 1379 |         landlock_log_denial(subject, &(struct landlock_request) {
      |                                                                 ^

We went 8 years before tripping over this! With this patch landed,
we can enable COMPILE_TEST builds with the randstruct GCC plugin again.

Reported-by: "Dr. David Alan Gilbert" <linux@treblig.org>
Closes: https://lore.kernel.org/lkml/Z_PRaKx7q70MKgCA@gallifrey/
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/lkml/20250407-kbuild-disable-gcc-plugins-v1-1-5d46ae583f5e@kernel.org/
Reported-by: WangYuli <wangyuli@uniontech.com>
Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Mickaël Salaün" <mic@digikod.net>
Cc: "Günther Noack" <gnoack@google.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: <linux-security-module@vger.kernel.org>
---
 security/landlock/fs.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 6fee7c20f64d..b2818afb0503 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -1376,14 +1376,14 @@ static void
 log_fs_change_topology_path(const struct landlock_cred_security *const subject,
 			    size_t handle_layer, const struct path *const path)
 {
-	landlock_log_denial(subject, &(struct landlock_request) {
+	struct landlock_request request = {
 		.type = LANDLOCK_REQUEST_FS_CHANGE_TOPOLOGY,
-		.audit = {
-			.type = LSM_AUDIT_DATA_PATH,
-			.u.path = *path,
-		},
+		.audit.type = LSM_AUDIT_DATA_PATH,
 		.layer_plus_one = handle_layer + 1,
-	});
+	};
+	request.audit.u.path = *path;
+
+	landlock_log_denial(subject, &request);
 }
 
 static void log_fs_change_topology_dentry(
@@ -1720,6 +1720,7 @@ static int hook_file_truncate(struct file *const file)
 static int hook_file_ioctl_common(const struct file *const file,
 				  const unsigned int cmd, const bool is_compat)
 {
+	struct lsm_ioctlop_audit audit_log;
 	access_mask_t allowed_access = landlock_file(file)->allowed_access;
 
 	/*
@@ -1738,14 +1739,13 @@ static int hook_file_ioctl_common(const struct file *const file,
 				  is_masked_device_ioctl(cmd))
 		return 0;
 
+	audit_log.path = file->f_path;
+	audit_log.cmd = cmd;
 	landlock_log_denial(landlock_cred(file->f_cred), &(struct landlock_request) {
 		.type = LANDLOCK_REQUEST_FS_ACCESS,
 		.audit = {
 			.type = LSM_AUDIT_DATA_IOCTL_OP,
-			.u.op = &(struct lsm_ioctlop_audit) {
-				.path = file->f_path,
-				.cmd = cmd,
-			},
+			.u.op = &audit_log,
 		},
 		.all_existing_optional_access = _LANDLOCK_ACCESS_FS_OPTIONAL,
 		.access = LANDLOCK_ACCESS_FS_IOCTL_DEV,
-- 
2.34.1

Re: [PATCH] landlock: Work around randstruct unnamed static initializer support

[Mickaël Salaün]

On Sun, Apr 20, 2025 at 05:08:59PM -0700, Kees Cook wrote:
> Unnamed static initializers aren't supported by the randstruct GCC
> plugin. Quoting the plugin, "set up a bogus anonymous struct field
> designed to error out on unnamed struct initializers as gcc provides
> no other way to detect such code". That is exactly what happens
> with the landlock code, so adjust the static initializers for structs
> lsm_ioctlop_audit and landlock_request that contain a randomized structure
> (struct path) to use named variables, which avoids the intentional
> GCC crashes:

This is not a sustainable solution.  Could we fix the plugin instead?
This new Landlock change may be the first to trigger this plugin bug but
it will probably not be the last to use unnamed static initializers.
Forbidding specific C constructs should be documented.

As a temporary solution (not sure if it's worth it, nor a good idea),
could handling of types used in unnamed static initializers be ignored
by this plugin?  I guess [1] is already a temporary solution though.
[1] https://lore.kernel.org/all/20250409151154.work.872-kees@kernel.org/

> 
> security/landlock/fs.c: In function 'hook_file_ioctl_common':
> security/landlock/fs.c:1745:61: internal compiler error: in count_type_elements, at expr.cc:7092
>  1745 |                         .u.op = &(struct lsm_ioctlop_audit) {
>       |                                                             ^
> 
> security/landlock/fs.c: In function 'log_fs_change_topology_path':
> security/landlock/fs.c:1379:65: internal compiler error: in count_type_elements, at expr.cc:7092
>  1379 |         landlock_log_denial(subject, &(struct landlock_request) {
>       |                                                                 ^
> 
> We went 8 years before tripping over this! With this patch landed,
> we can enable COMPILE_TEST builds with the randstruct GCC plugin again.
> 
> Reported-by: "Dr. David Alan Gilbert" <linux@treblig.org>
> Closes: https://lore.kernel.org/lkml/Z_PRaKx7q70MKgCA@gallifrey/
> Reported-by: Mark Brown <broonie@kernel.org>
> Closes: https://lore.kernel.org/lkml/20250407-kbuild-disable-gcc-plugins-v1-1-5d46ae583f5e@kernel.org/
> Reported-by: WangYuli <wangyuli@uniontech.com>
> Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
> Cc: "Mickaël Salaün" <mic@digikod.net>
> Cc: "Günther Noack" <gnoack@google.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Paul Moore <paul@paul-moore.com>
> Cc: James Morris <jmorris@namei.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>
> Cc: <linux-security-module@vger.kernel.org>
> ---
>  security/landlock/fs.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index 6fee7c20f64d..b2818afb0503 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -1376,14 +1376,14 @@ static void
>  log_fs_change_topology_path(const struct landlock_cred_security *const subject,
>  			    size_t handle_layer, const struct path *const path)
>  {
> -	landlock_log_denial(subject, &(struct landlock_request) {
> +	struct landlock_request request = {
>  		.type = LANDLOCK_REQUEST_FS_CHANGE_TOPOLOGY,
> -		.audit = {
> -			.type = LSM_AUDIT_DATA_PATH,
> -			.u.path = *path,
> -		},
> +		.audit.type = LSM_AUDIT_DATA_PATH,
>  		.layer_plus_one = handle_layer + 1,
> -	});
> +	};
> +	request.audit.u.path = *path;
> +
> +	landlock_log_denial(subject, &request);
>  }
>  
>  static void log_fs_change_topology_dentry(
> @@ -1720,6 +1720,7 @@ static int hook_file_truncate(struct file *const file)
>  static int hook_file_ioctl_common(const struct file *const file,
>  				  const unsigned int cmd, const bool is_compat)
>  {
> +	struct lsm_ioctlop_audit audit_log;
>  	access_mask_t allowed_access = landlock_file(file)->allowed_access;
>  
>  	/*
> @@ -1738,14 +1739,13 @@ static int hook_file_ioctl_common(const struct file *const file,
>  				  is_masked_device_ioctl(cmd))
>  		return 0;
>  
> +	audit_log.path = file->f_path;
> +	audit_log.cmd = cmd;
>  	landlock_log_denial(landlock_cred(file->f_cred), &(struct landlock_request) {
>  		.type = LANDLOCK_REQUEST_FS_ACCESS,
>  		.audit = {
>  			.type = LSM_AUDIT_DATA_IOCTL_OP,
> -			.u.op = &(struct lsm_ioctlop_audit) {
> -				.path = file->f_path,
> -				.cmd = cmd,
> -			},
> +			.u.op = &audit_log,
>  		},
>  		.all_existing_optional_access = _LANDLOCK_ACCESS_FS_OPTIONAL,
>  		.access = LANDLOCK_ACCESS_FS_IOCTL_DEV,
> -- 
> 2.34.1
> 
> 

Re: [PATCH] landlock: Work around randstruct unnamed static initializer support

[Arnd Bergmann]

On Tue, Apr 22, 2025, at 14:25, Mickaël Salaün wrote:
> On Sun, Apr 20, 2025 at 05:08:59PM -0700, Kees Cook wrote:
>> Unnamed static initializers aren't supported by the randstruct GCC
>> plugin. Quoting the plugin, "set up a bogus anonymous struct field
>> designed to error out on unnamed struct initializers as gcc provides
>> no other way to detect such code". That is exactly what happens
>> with the landlock code, so adjust the static initializers for structs
>> lsm_ioctlop_audit and landlock_request that contain a randomized structure
>> (struct path) to use named variables, which avoids the intentional
>> GCC crashes:
>
> This is not a sustainable solution.  Could we fix the plugin instead?
> This new Landlock change may be the first to trigger this plugin bug but
> it will probably not be the last to use unnamed static initializers.
> Forbidding specific C constructs should be documented.

I think the version from Kees' patch looks more readable than
the version with the compound literal, so it certainly seems appropriate
as an immediate regression fix, even if it's possible to fix the
plugin later.

>> We went 8 years before tripping over this! 

Right, it's probably enough to revisit the plugin code after
it happens again.

>> Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
>> Signed-off-by: Kees Cook <kees@kernel.org>

Acked-by: Arnd Bergmann <arnd@arndb.de>

Re: [PATCH] landlock: Work around randstruct unnamed static initializer support

[Kees Cook]

On Tue, Apr 22, 2025 at 02:53:05PM +0200, Arnd Bergmann wrote:
> On Tue, Apr 22, 2025, at 14:25, Mickaël Salaün wrote:
> > On Sun, Apr 20, 2025 at 05:08:59PM -0700, Kees Cook wrote:
> >> Unnamed static initializers aren't supported by the randstruct GCC
> >> plugin. Quoting the plugin, "set up a bogus anonymous struct field
> >> designed to error out on unnamed struct initializers as gcc provides
> >> no other way to detect such code". That is exactly what happens
> >> with the landlock code, so adjust the static initializers for structs
> >> lsm_ioctlop_audit and landlock_request that contain a randomized structure
> >> (struct path) to use named variables, which avoids the intentional
> >> GCC crashes:
> >
> > This is not a sustainable solution.  Could we fix the plugin instead?
> > This new Landlock change may be the first to trigger this plugin bug but
> > it will probably not be the last to use unnamed static initializers.
> > Forbidding specific C constructs should be documented.
> 
> I think the version from Kees' patch looks more readable than
> the version with the compound literal, so it certainly seems appropriate
> as an immediate regression fix, even if it's possible to fix the
> plugin later.
> 
> >> We went 8 years before tripping over this! 
> 
> Right, it's probably enough to revisit the plugin code after
> it happens again.

Yeah, that's my thinking as well.

> >> Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
> >> Signed-off-by: Kees Cook <kees@kernel.org>
> 
> Acked-by: Arnd Bergmann <arnd@arndb.de>

Thanks! Mickaël, are you good with this for now, and if so, do you want
to carry it or shall I?

-Kees

-- 
Kees Cook

Re: [PATCH] landlock: Work around randstruct unnamed static initializer support

[Mickaël Salaün]

On Tue, Apr 22, 2025 at 07:59:07AM -0700, Kees Cook wrote:
> On Tue, Apr 22, 2025 at 02:53:05PM +0200, Arnd Bergmann wrote:
> > On Tue, Apr 22, 2025, at 14:25, Mickaël Salaün wrote:
> > > On Sun, Apr 20, 2025 at 05:08:59PM -0700, Kees Cook wrote:
> > >> Unnamed static initializers aren't supported by the randstruct GCC
> > >> plugin. Quoting the plugin, "set up a bogus anonymous struct field
> > >> designed to error out on unnamed struct initializers as gcc provides
> > >> no other way to detect such code". That is exactly what happens
> > >> with the landlock code, so adjust the static initializers for structs
> > >> lsm_ioctlop_audit and landlock_request that contain a randomized structure
> > >> (struct path) to use named variables, which avoids the intentional
> > >> GCC crashes:
> > >
> > > This is not a sustainable solution.  Could we fix the plugin instead?
> > > This new Landlock change may be the first to trigger this plugin bug but
> > > it will probably not be the last to use unnamed static initializers.
> > > Forbidding specific C constructs should be documented.
> > 
> > I think the version from Kees' patch looks more readable than
> > the version with the compound literal, so it certainly seems appropriate
> > as an immediate regression fix, even if it's possible to fix the
> > plugin later.
> > 
> > >> We went 8 years before tripping over this! 
> > 
> > Right, it's probably enough to revisit the plugin code after
> > it happens again.
> 
> Yeah, that's my thinking as well.
> 
> > >> Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
> > >> Signed-off-by: Kees Cook <kees@kernel.org>
> > 
> > Acked-by: Arnd Bergmann <arnd@arndb.de>
> 
> Thanks! Mickaël, are you good with this for now, and if so, do you want
> to carry it or shall I?

I don't like this kind of change but I'm OK with it if it comes with
documentation explaining what construct should not be used in the kernel
(and even better, updating checkpatch.pl accordingly), or if it's a
temporary workaround and someone is working on fixing the plugin.  Are
you or someone else working on it?  In the meantime, feel free to carry
this patch.

Re: [PATCH] landlock: Work around randstruct unnamed static initializer support

[Günther Noack]

Hello Kees!

On Sun, Apr 20, 2025 at 05:08:59PM -0700, Kees Cook wrote:
> Unnamed static initializers aren't supported by the randstruct GCC
> plugin. Quoting the plugin, "set up a bogus anonymous struct field
> designed to error out on unnamed struct initializers as gcc provides
> no other way to detect such code". That is exactly what happens
> with the landlock code, so adjust the static initializers for structs
> lsm_ioctlop_audit and landlock_request that contain a randomized structure
> (struct path) to use named variables, which avoids the intentional
> GCC crashes:
> 
> security/landlock/fs.c: In function 'hook_file_ioctl_common':
> security/landlock/fs.c:1745:61: internal compiler error: in count_type_elements, at expr.cc:7092
>  1745 |                         .u.op = &(struct lsm_ioctlop_audit) {
>       |                                                             ^
> 
> security/landlock/fs.c: In function 'log_fs_change_topology_path':
> security/landlock/fs.c:1379:65: internal compiler error: in count_type_elements, at expr.cc:7092
>  1379 |         landlock_log_denial(subject, &(struct landlock_request) {
>       |                                                                 ^
> 
> We went 8 years before tripping over this! With this patch landed,
> we can enable COMPILE_TEST builds with the randstruct GCC plugin again.

I am still struggling to understand the boundaries of what the randstruct plugin
supports and what it fails on.  Could you please clarify?  (Specific questions
below.)


> Reported-by: "Dr. David Alan Gilbert" <linux@treblig.org>
> Closes: https://lore.kernel.org/lkml/Z_PRaKx7q70MKgCA@gallifrey/
> Reported-by: Mark Brown <broonie@kernel.org>
> Closes: https://lore.kernel.org/lkml/20250407-kbuild-disable-gcc-plugins-v1-1-5d46ae583f5e@kernel.org/
> Reported-by: WangYuli <wangyuli@uniontech.com>
> Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-7809355f1381@uniontech.com/
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
> Cc: "Mickaël Salaün" <mic@digikod.net>
> Cc: "Günther Noack" <gnoack@google.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Paul Moore <paul@paul-moore.com>
> Cc: James Morris <jmorris@namei.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>
> Cc: <linux-security-module@vger.kernel.org>
> ---
>  security/landlock/fs.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index 6fee7c20f64d..b2818afb0503 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -1376,14 +1376,14 @@ static void
>  log_fs_change_topology_path(const struct landlock_cred_security *const subject,
>  			    size_t handle_layer, const struct path *const path)
>  {
> -	landlock_log_denial(subject, &(struct landlock_request) {
> +	struct landlock_request request = {
>  		.type = LANDLOCK_REQUEST_FS_CHANGE_TOPOLOGY,
> -		.audit = {
> -			.type = LSM_AUDIT_DATA_PATH,
> -			.u.path = *path,
> -		},
> +		.audit.type = LSM_AUDIT_DATA_PATH,
>  		.layer_plus_one = handle_layer + 1,
> -	});
> +	};
> +	request.audit.u.path = *path;
> +
> +	landlock_log_denial(subject, &request);
>  }

If I understood the commit message correctly, we are giving a name ("request")
to the struct landlock_request here, because the randstruct plugin needs that if
the constructed struct (recursively?) contains a randstruct-enabled member, like
.audit.u.path in this case?

I am surprised that you pulled the assignment to "request.audit.u.path" out of
the struct initialization expression though.  Was that also necessary for
randstruct to work?

Would it have worked to initialize it inline instead, like this?

	struct landlock_request request = {
		.type           = LANDLOCK_REQUEST_FS_CHANGE_TOPOLOGY,
		.audit.type     = LSM_AUDIT_DATA_PATH,
    .audit.u.path   = *path,
		.layer_plus_one = handle_layer + 1,
	};


>  
>  static void log_fs_change_topology_dentry(
> @@ -1720,6 +1720,7 @@ static int hook_file_truncate(struct file *const file)
>  static int hook_file_ioctl_common(const struct file *const file,
>  				  const unsigned int cmd, const bool is_compat)
>  {
> +	struct lsm_ioctlop_audit audit_log;
>  	access_mask_t allowed_access = landlock_file(file)->allowed_access;
>  
>  	/*
> @@ -1738,14 +1739,13 @@ static int hook_file_ioctl_common(const struct file *const file,
>  				  is_masked_device_ioctl(cmd))
>  		return 0;
>  
> +	audit_log.path = file->f_path;
> +	audit_log.cmd = cmd;

Same question here, I guess. This could not have been written like this?

  struct lsm_ioctlop_audit audit_log = {
    .path = file->f_path,
    .cmd  = cmd,
  };


>  	landlock_log_denial(landlock_cred(file->f_cred), &(struct landlock_request) {
>  		.type = LANDLOCK_REQUEST_FS_ACCESS,
>  		.audit = {
>  			.type = LSM_AUDIT_DATA_IOCTL_OP,
> -			.u.op = &(struct lsm_ioctlop_audit) {
> -				.path = file->f_path,
> -				.cmd = cmd,
> -			},
> +			.u.op = &audit_log,
>  		},
>  		.all_existing_optional_access = _LANDLOCK_ACCESS_FS_OPTIONAL,
>  		.access = LANDLOCK_ACCESS_FS_IOCTL_DEV,
> -- 
> 2.34.1
> 

That being said, the code transformation in this patch is obviously correct, and
it's good to fix the build, so looks good:

Reviewed-by: Günther Noack <gnoack@google.com>

But as Mickaël also said, it seems worrying that otherwise correct C code does
not compile with that plugin, especially when the conditions under which that
happens are not clear.

I would also like to understand this better, so that we can avoid tripping over
it in the future.

Thanks!
-Günther