Re: [PATCH][next] xfs: Replace one-element arrays with flexible-array members
From: Dave Chinner
Date: Sun Feb 05 2023 - 17:51:27 EST
On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
> One-element arrays are deprecated, and we are replacing them with flexible
> array members instead. So, replace one-element arrays with flexible-array
> members in structures xfs_attr_leaf_name_local and
> xfs_attr_leaf_name_remote.
>
> The only binary differences reported after the changes are all like
> these:
>
> fs/xfs/libxfs/xfs_attr_leaf.o
> _@@ -435,7 +435,7 @@
> 3b8: movzbl 0x2(%rbx),%eax
> 3bc: rol $0x8,%bp
> 3c0: movzwl %bp,%ebp
> - 3c3: lea 0x2(%rax,%rbp,1),%ebx
> + 3c3: lea 0x3(%rax,%rbp,1),%ebx
> 3c7: call 3cc <xfs_attr_leaf_entsize+0x8c>
> 3c8: R_X86_64_PLT32 __tsan_func_exit-0x4
> 3cc: or $0x3,%ebx
> _@@ -454,7 +454,7 @@
> 3ea: movzbl 0x8(%rbx),%ebx
> 3ee: call 3f3 <xfs_attr_leaf_entsize+0xb3>
> 3ef: R_X86_64_PLT32 __tsan_func_exit-0x4
> - 3f3: add $0xa,%ebx
> + 3f3: add $0xb,%ebx
> 3f6: or $0x3,%ebx
> 3f9: add $0x1,%ebx
> 3fc: mov %ebx,%eax
>
> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
That seems like a red flag to me - an off-by-one change in the
compiled code that calculates of the on-disk size of a structure as
a result of an in-memory structure change just smells like a bug.
How did you test this change?
> And the reason for this is because of the round_up() macro called in
> functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
> which is compensanting for the one-byte reduction in size (due to the
> flex-array transformation) of structures xfs_attr_leaf_name_remote and
> xfs_attr_leaf_name_local. So, sizes remain the same before and after
> changes.
I'm not sure that is true. Before this change:
sizeof(xfs_attr_leaf_name_local_t) = 4
sizeof(xfs_attr_leaf_name_remote_t) = 12
After this change:
sizeof(xfs_attr_leaf_name_local_t) = 4
sizeof(xfs_attr_leaf_name_remote_t) = 12
i.e. no change because the structures aren't defined as packed
structures. Hence the compiler pads them to out to 4 byte alignment
naturally regardless of the flex array definition. pahole on x86-64
also confirms that the (padded) size of the structure is not
changed.
However, the on-disk structure it is being used to decode is packed,
and we're only using pointer arithmetic to pull the location of the
name/value pairs out of the buffer to copy them - it's the structure
size calculations that actually define the size of the structures
for a given name length, not the sizeof() value or the flex array
definitions...
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
>
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/251
> Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> ---
> fs/xfs/libxfs/xfs_da_format.h | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
> index 25e2841084e1..e1e62ebb0c44 100644
> --- a/fs/xfs/libxfs/xfs_da_format.h
> +++ b/fs/xfs/libxfs/xfs_da_format.h
> @@ -620,14 +620,14 @@ typedef struct xfs_attr_leaf_entry { /* sorted on key, not name */
> typedef struct xfs_attr_leaf_name_local {
> __be16 valuelen; /* number of bytes in value */
> __u8 namelen; /* length of name bytes */
> - __u8 nameval[1]; /* name/value bytes */
> + __u8 nameval[]; /* name/value bytes */
> } xfs_attr_leaf_name_local_t;
>
> typedef struct xfs_attr_leaf_name_remote {
> __be32 valueblk; /* block number of value bytes */
> __be32 valuelen; /* number of bytes in value */
> __u8 namelen; /* length of name bytes */
> - __u8 name[1]; /* name bytes */
> + __u8 name[]; /* name bytes */
> } xfs_attr_leaf_name_remote_t;
>
> typedef struct xfs_attr_leafblock {
> @@ -747,13 +747,13 @@ xfs_attr3_leaf_name_local(xfs_attr_leafblock_t *leafp, int idx)
> */
> static inline int xfs_attr_leaf_entsize_remote(int nlen)
> {
> - return round_up(sizeof(struct xfs_attr_leaf_name_remote) - 1 +
> + return round_up(sizeof(struct xfs_attr_leaf_name_remote) +
> nlen, XFS_ATTR_LEAF_NAME_ALIGN);
> }
To be honest, the actual padding and alignment calculations are
kinda whacky because that's the way they were defined back in 1995.
And, well, once set in the on-disk format, it can't easily be
changed. FYI, here's the original definition from 1995:
#define XFS_ATTR_LEAF_ENTSIZE_REMOTE(nlen) /* space for remote struct */ \
(((sizeof(xfs_attr_leaf_name_remote_t)-1 + (nlen)) +3)&~0x3)
So apart using round_up and defines instead of magic numbers, the
current calculation is unchanged from the original definition.
AFAICT, the modification you are proposing above breaks this because the
sizeof(xfs_attr_leaf_name_remote) result has not changed with the
change of the structure definition.
e.g. if namelen = 17, before we had:
size = round_up(12 - 1 + 17, 4)
= round_up(28, 4)
= 28
Which is correct because the on-disk format is packed:
0 4 89 12 20 26 28
+---+---++--+-------+-----+-+-----....
|---------------| 17 bytes of name.
|-| 2 bytes of padding
|-----.... Next attr record.
We end up with 2 bytes of padded between the end of the name and the
start of the next attribute record in the block.
But after this patch, now we calculate the size as:
size = round_up(12 + 17, 4)
= round_up(29, 4)
= 32
Which is a different result, and would result in incorrect parsing
of the attribute records in the buffer. Hence I don't think it is
valid to be changing the entsize calculations like this if sizeof()
is not changing results.
Which comes back to my original question: how did you test this?
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx