[PATCH][next] fs: omfs: Use flexible-array member in struct omfs_extent

[PATCH][next] fs: omfs: Use flexible-array member in struct omfs_extent

[Gustavo A. R. Silva]

Memory for 'struct omfs_extent' and a 'e_extent_count' number of extent
entries is indirectly allocated through 'bh->b_data', which is a pointer
to data within the page. This implies that the member 'e_entry'
(which is the start of extent entries) functions more like an array than
a single object of type 'struct omfs_extent_entry'.

So we better turn this object into a proper array, in this case a
flexible-array member, and with that, fix the following
-Wstringop-overflow warning seen after building s390 architecture with
allyesconfig (GCC 13):

fs/omfs/file.c: In function 'omfs_grow_extent':
include/linux/fortify-string.h:57:33: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
   57 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
include/linux/fortify-string.h:648:9: note: in expansion of macro '__underlying_memcpy'
  648 |         __underlying_##op(p, q, __fortify_size);                        \
      |         ^~~~~~~~~~~~~
include/linux/fortify-string.h:693:26: note: in expansion of macro '__fortify_memcpy_chk'
  693 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
      |                          ^~~~~~~~~~~~~~~~~~~~
fs/omfs/file.c:170:9: note: in expansion of macro 'memcpy'
  170 |         memcpy(terminator, entry, sizeof(struct omfs_extent_entry));
      |         ^~~~~~
In file included from fs/omfs/omfs.h:8,
                 from fs/omfs/file.c:11:
fs/omfs/omfs_fs.h:80:34: note: at offset 16 into destination object 'e_entry' of size 16
   80 |         struct omfs_extent_entry e_entry;       /* start of extent entries */
      |                                  ^~~~~~~

There are some binary differences before and after changes, but this are
expected due to the change in the size of 'struct omfs_extent' and the
necessary adjusments.

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: https://github.com/KSPP/linux/issues/330
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 fs/omfs/file.c    | 12 ++++++------
 fs/omfs/omfs_fs.h |  2 +-
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/omfs/file.c b/fs/omfs/file.c
index de8f57ee39ec..6b580b9da8e3 100644
--- a/fs/omfs/file.c
+++ b/fs/omfs/file.c
@@ -14,7 +14,7 @@ static u32 omfs_max_extents(struct omfs_sb_info *sbi, int offset)
 {
 	return (sbi->s_sys_blocksize - offset -
 		sizeof(struct omfs_extent)) /
-		sizeof(struct omfs_extent_entry) + 1;
+		sizeof(struct omfs_extent_entry);
 }
 
 void omfs_make_empty_table(struct buffer_head *bh, int offset)
@@ -24,8 +24,8 @@ void omfs_make_empty_table(struct buffer_head *bh, int offset)
 	oe->e_next = ~cpu_to_be64(0ULL);
 	oe->e_extent_count = cpu_to_be32(1),
 	oe->e_fill = cpu_to_be32(0x22),
-	oe->e_entry.e_cluster = ~cpu_to_be64(0ULL);
-	oe->e_entry.e_blocks = ~cpu_to_be64(0ULL);
+	oe->e_entry[0].e_cluster = ~cpu_to_be64(0ULL);
+	oe->e_entry[0].e_blocks = ~cpu_to_be64(0ULL);
 }
 
 int omfs_shrink_inode(struct inode *inode)
@@ -68,7 +68,7 @@ int omfs_shrink_inode(struct inode *inode)
 
 		last = next;
 		next = be64_to_cpu(oe->e_next);
-		entry = &oe->e_entry;
+		entry = oe->e_entry;
 
 		/* ignore last entry as it is the terminator */
 		for (; extent_count > 1; extent_count--) {
@@ -117,7 +117,7 @@ static int omfs_grow_extent(struct inode *inode, struct omfs_extent *oe,
 			u64 *ret_block)
 {
 	struct omfs_extent_entry *terminator;
-	struct omfs_extent_entry *entry = &oe->e_entry;
+	struct omfs_extent_entry *entry = oe->e_entry;
 	struct omfs_sb_info *sbi = OMFS_SB(inode->i_sb);
 	u32 extent_count = be32_to_cpu(oe->e_extent_count);
 	u64 new_block = 0;
@@ -245,7 +245,7 @@ static int omfs_get_block(struct inode *inode, sector_t block,
 
 		extent_count = be32_to_cpu(oe->e_extent_count);
 		next = be64_to_cpu(oe->e_next);
-		entry = &oe->e_entry;
+		entry = oe->e_entry;
 
 		if (extent_count > max_extents)
 			goto out_brelse;
diff --git a/fs/omfs/omfs_fs.h b/fs/omfs/omfs_fs.h
index caecb3d5a344..1ff6b9e41297 100644
--- a/fs/omfs/omfs_fs.h
+++ b/fs/omfs/omfs_fs.h
@@ -77,7 +77,7 @@ struct omfs_extent {
 	__be64 e_next;			/* next extent table location */
 	__be32 e_extent_count;		/* total # extents in this table */
 	__be32 e_fill;
-	struct omfs_extent_entry e_entry;	/* start of extent entries */
+	struct omfs_extent_entry e_entry[];	/* start of extent entries */
 };
 
 #endif
-- 
2.34.1

Re: [PATCH][next] fs: omfs: Use flexible-array member in struct omfs_extent

[Kees Cook]

On Fri, Jul 14, 2023 at 01:56:37PM -0600, Gustavo A. R. Silva wrote:
> There are some binary differences before and after changes, but this are
> expected due to the change in the size of 'struct omfs_extent' and the
> necessary adjusments.

For binary changes, I think commit logs should have more details. In
this case, I can figure it out:

> diff --git a/fs/omfs/file.c b/fs/omfs/file.c
> index de8f57ee39ec..6b580b9da8e3 100644
> --- a/fs/omfs/file.c
> +++ b/fs/omfs/file.c
> @@ -14,7 +14,7 @@ static u32 omfs_max_extents(struct omfs_sb_info *sbi, int offset)
>  {
>  	return (sbi->s_sys_blocksize - offset -
>  		sizeof(struct omfs_extent)) /
> -		sizeof(struct omfs_extent_entry) + 1;
> +		sizeof(struct omfs_extent_entry);
>  }

I think the original calculation meant to do:

  	return (sbi->s_sys_blocksize - offset -
  		(sizeof(struct omfs_extent) - sizeof(struct omfs_extent_entry))) /
		sizeof(struct omfs_extent_entry);

So this binary difference looks correct. I rebuilt before/after this
patch with omfs_max_extents() marked as noinline, and all the binary
changes were isolated here, and did exactly as expected: the first half
is 16 smaller (size of struct omfs_extent_entry), and the final +1 is
removed:

-     2e1:      lea    -0x20(%rbx),%rax
+     2e1:      lea    -0x10(%rbx),%rax
      2e5:      pop    %rbx
      2e6:      pop    %rbp
      2e7:      shr    $0x4,%rax
-     2eb:      add    $0x1,%eax

So this looks correct to me. Thanks!

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

Re: [PATCH][next] fs: omfs: Use flexible-array member in struct omfs_extent

[Bob Copeland]

On Mon, Jul 17, 2023 at 08:20:22AM -0700, Kees Cook wrote:
> > @@ -14,7 +14,7 @@ static u32 omfs_max_extents(struct omfs_sb_info *sbi, int offset)
> >  {
> >  	return (sbi->s_sys_blocksize - offset -
> >  		sizeof(struct omfs_extent)) /
> > -		sizeof(struct omfs_extent_entry) + 1;
> > +		sizeof(struct omfs_extent_entry);
> >  }
> 
> I think the original calculation meant to do:
> 
>   	return (sbi->s_sys_blocksize - offset -
>   		(sizeof(struct omfs_extent) - sizeof(struct omfs_extent_entry))) /
> 		sizeof(struct omfs_extent_entry);

I can confirm the intent.  I also went back and looked at the usages just
to be sure.

In this case, there are two possible values for x = s_sys_blocksize - offset:
7728 or 8128, and both can be divided by 16 evenly so you have either:

    f_old = (x - 32) / 16 + 1 or:
    f_new = (x - 16) / 16

they both simplify to x/16 - 1 so they are equivalent for these inputs.

-- 
Bob Copeland %% https://bobcopeland.com/