[PATCH] audit: Replace zero-length array with flexible-array

[PATCH] audit: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 include/linux/audit.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/audit.h b/include/linux/audit.h
index f9ceae57ca8d..2b63aee6e9fa 100644
--- a/include/linux/audit.h
+++ b/include/linux/audit.h
@@ -19,7 +19,7 @@
 struct audit_sig_info {
 	uid_t		uid;
 	pid_t		pid;
-	char		ctx[0];
+	char		ctx[];
 };
 
 struct audit_buffer;

Re: [PATCH] audit: Replace zero-length array with flexible-array

[Richard Guy Briggs]

On 2020-05-07 13:50, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
> 
> struct foo {
>         int stuff;
>         struct boo array[];
> };
> 
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
> 
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
> 
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
> 
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
> 
> This issue was found with the help of Coccinelle.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Sounds reasonable to me.  There's another in include/uapi/linux/audit.h
in struct audit_rule_data buf[0].  This alert also helped me fix another
one in a patchset I'm about to post (and will probably cause a merge
conflict but we can figure that out).

Reviewed-by: Richard Guy Briggs <rgb@redhat.com>

> ---
>  include/linux/audit.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index f9ceae57ca8d..2b63aee6e9fa 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -19,7 +19,7 @@
>  struct audit_sig_info {
>  	uid_t		uid;
>  	pid_t		pid;
> -	char		ctx[0];
> +	char		ctx[];
>  };
>  
>  struct audit_buffer;
> 
> 
> --
> Linux-audit mailing list
> Linux-audit@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-audit

- RGB

--
Richard Guy Briggs <rgb@redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635

Re: [PATCH] audit: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

On Thu, May 07, 2020 at 05:58:13PM -0400, Richard Guy Briggs wrote:
> On 2020-05-07 13:50, Gustavo A. R. Silva wrote:
> > The current codebase makes use of the zero-length array language
> > extension to the C90 standard, but the preferred mechanism to declare
> > variable-length types such as these ones is a flexible array member[1][2],
> > introduced in C99:
> > 
> > struct foo {
> >         int stuff;
> >         struct boo array[];
> > };
> > 
> > By making use of the mechanism above, we will get a compiler warning
> > in case the flexible array does not occur last in the structure, which
> > will help us prevent some kind of undefined behavior bugs from being
> > inadvertently introduced[3] to the codebase from now on.
> > 
> > Also, notice that, dynamic memory allocations won't be affected by
> > this change:
> > 
> > "Flexible array members have incomplete type, and so the sizeof operator
> > may not be applied. As a quirk of the original implementation of
> > zero-length arrays, sizeof evaluates to zero."[1]
> > 
> > sizeof(flexible-array-member) triggers a warning because flexible array
> > members have incomplete type[1]. There are some instances of code in
> > which the sizeof operator is being incorrectly/erroneously applied to
> > zero-length arrays and the result is zero. Such instances may be hiding
> > some bugs. So, this work (flexible-array member conversions) will also
> > help to get completely rid of those sorts of issues.
> > 
> > This issue was found with the help of Coccinelle.
> > 
> > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > [2] https://github.com/KSPP/linux/issues/21
> > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> 
> Sounds reasonable to me.  There's another in include/uapi/linux/audit.h

Hi,

I wouldn't advise to make any of these conversions in include/uapi/
[1][2].

> in struct audit_rule_data buf[0].  This alert also helped me fix another
> one in a patchset I'm about to post (and will probably cause a merge
> conflict but we can figure that out).

Awesome. :)

> 
> Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
>

Thanks
--
Gustavo

[1] https://lore.kernel.org/lkml/20200424121553.GE26002@ziepe.ca/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1e6e9d0f4859ec698d55381ea26f4136eff3afe1

> > ---
> >  include/linux/audit.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > index f9ceae57ca8d..2b63aee6e9fa 100644
> > --- a/include/linux/audit.h
> > +++ b/include/linux/audit.h
> > @@ -19,7 +19,7 @@
> >  struct audit_sig_info {
> >  	uid_t		uid;
> >  	pid_t		pid;
> > -	char		ctx[0];
> > +	char		ctx[];
> >  };
> >  
> >  struct audit_buffer;
> > 
> > 
> > --
> > Linux-audit mailing list
> > Linux-audit@redhat.com
> > https://www.redhat.com/mailman/listinfo/linux-audit
> 
> - RGB
> 
> --
> Richard Guy Briggs <rgb@redhat.com>
> Sr. S/W Engineer, Kernel Security, Base Operating Systems
> Remote, Ottawa, Red Hat Canada
> IRC: rgb, SunRaycer
> Voice: +1.647.777.2635, Internal: (81) 32635
> 

Re: [PATCH] audit: Replace zero-length array with flexible-array

[Paul Moore]

On Thu, May 7, 2020 at 6:45 PM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
> I wouldn't advise to make any of these conversions in include/uapi/ ...

Yes, let's not make changes like this to anything under include/uapi;
the potential reward doesn't outweigh the risks.

-- 
paul moore
www.paul-moore.com

Re: [PATCH] audit: Replace zero-length array with flexible-array

[Richard Guy Briggs]

On 2020-05-07 17:49, Gustavo A. R. Silva wrote:
> On Thu, May 07, 2020 at 05:58:13PM -0400, Richard Guy Briggs wrote:
> > On 2020-05-07 13:50, Gustavo A. R. Silva wrote:
> > > The current codebase makes use of the zero-length array language
> > > extension to the C90 standard, but the preferred mechanism to declare
> > > variable-length types such as these ones is a flexible array member[1][2],
> > > introduced in C99:
> > > 
> > > struct foo {
> > >         int stuff;
> > >         struct boo array[];
> > > };
> > > 
> > > By making use of the mechanism above, we will get a compiler warning
> > > in case the flexible array does not occur last in the structure, which
> > > will help us prevent some kind of undefined behavior bugs from being
> > > inadvertently introduced[3] to the codebase from now on.
> > > 
> > > Also, notice that, dynamic memory allocations won't be affected by
> > > this change:
> > > 
> > > "Flexible array members have incomplete type, and so the sizeof operator
> > > may not be applied. As a quirk of the original implementation of
> > > zero-length arrays, sizeof evaluates to zero."[1]
> > > 
> > > sizeof(flexible-array-member) triggers a warning because flexible array
> > > members have incomplete type[1]. There are some instances of code in
> > > which the sizeof operator is being incorrectly/erroneously applied to
> > > zero-length arrays and the result is zero. Such instances may be hiding
> > > some bugs. So, this work (flexible-array member conversions) will also
> > > help to get completely rid of those sorts of issues.
> > > 
> > > This issue was found with the help of Coccinelle.
> > > 
> > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > > [2] https://github.com/KSPP/linux/issues/21
> > > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > > 
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > 
> > Sounds reasonable to me.  There's another in include/uapi/linux/audit.h
> 
> Hi,

Hello Gustavo,

> I wouldn't advise to make any of these conversions in include/uapi/
> [1][2].

Ah-hah.  Thanks for the contra-indicating supporting references.

> > in struct audit_rule_data buf[0].  This alert also helped me fix another
> > one in a patchset I'm about to post (and will probably cause a merge
> > conflict but we can figure that out).
> 
> Awesome. :)
> 
> > Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
> 
> Thanks
> --
> Gustavo
> 
> [1] https://lore.kernel.org/lkml/20200424121553.GE26002@ziepe.ca/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1e6e9d0f4859ec698d55381ea26f4136eff3afe1
> 
> > > ---
> > >  include/linux/audit.h |    2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > > index f9ceae57ca8d..2b63aee6e9fa 100644
> > > --- a/include/linux/audit.h
> > > +++ b/include/linux/audit.h
> > > @@ -19,7 +19,7 @@
> > >  struct audit_sig_info {
> > >  	uid_t		uid;
> > >  	pid_t		pid;
> > > -	char		ctx[0];
> > > +	char		ctx[];
> > >  };
> > >  
> > >  struct audit_buffer;
> > 
> > - RGB

- RGB

--
Richard Guy Briggs <rgb@redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635

Re: [PATCH] audit: Replace zero-length array with flexible-array

[Paul Moore]

On Thu, May 7, 2020 at 2:46 PM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
>         int stuff;
>         struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  include/linux/audit.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Merged into audit/next, thanks!

> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index f9ceae57ca8d..2b63aee6e9fa 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -19,7 +19,7 @@
>  struct audit_sig_info {
>         uid_t           uid;
>         pid_t           pid;
> -       char            ctx[0];
> +       char            ctx[];
>  };
>
>  struct audit_buffer;

-- 
paul moore
www.paul-moore.com

Re: [PATCH] audit: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

On Thu, May 07, 2020 at 10:52:09PM -0400, Paul Moore wrote:
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > ---
> >  include/linux/audit.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Merged into audit/next, thanks!
> 

Thanks, Paul.

--
Gustavo