Re: [PATCH][next] xfs: Replace one-element arrays with flexible-array members
From: Gustavo A. R. Silva
Date: Sun Feb 05 2023 - 19:21:10 EST
On 2/5/23 16:51, Dave Chinner wrote:
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.
Ughh..
You're right. I somehow got confused between the moment I first
build-tested this in my build machine and after a final last-minute
review I did on the machine from which I ultimately send the patches
out.
More comments below...
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:
Yeah; this in fact was a final last-minute review I did before sending out
the patch, and it was when I noticed the round_up() macro was doing something
quite idiomatic when it comes to calculating the sizes of structures containing
one-element arrays. People usually subtract the sizeof(type-of-one-element)
from the sizeof(struct-with-one-element-array) when they perform other
calculations. And in this case as the sizeof(type-of-one-element) is one byte,
at the moment I thought that subtraction was because of that, and then when I
build-tested that final change, I totally forgot about the padding (I had
actually noticed it when I modified the structure definitions :/) and now I
see I got all confused.
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
Yes; in fact I noticed that. :/
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.
Yep; I actually was going to include the pahole output for both structures
in the changelog text, but I decided not to do it at the last minute as
I didn't see it necessary because, as you pointed out, the sizes before
and after the flex-array transformations are the same.
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.
Yep; you're right.
Which comes back to my original question: how did you test this?
I compared the generated object files in fs/xfs/, fs/xfs/scrub/ and
fs/xfs/libxfs/ before and after the changes with something like
these[1]:
ARGS=--disassemble --demangle --reloc --no-show-raw-insn --section=.text
for i in $(cd $OUT/xfs/before && echo *.o); do echo $i; diff -u <(objdump $ARGS $OUT/xfs/before/$i | sed "0,/^Disassembly/d") <(objdump $ARGS $OUT/xfs/after/$i
| sed "0,/^Disassembly/d"); done
where of course the generated object files before the changes are
located in OUT/xfs/before/ and the ones after changes in $OUT/xfs/after/
I just double-checked and, indeed, the changes I mentioned in the
changelog text only show up when I modify the entsize functions.
So, because of the padding, the flex-array transformations don't
actually affect the sizes of the involved structures. So, it seems
that change is enough and is the correct one.
I really appreciate your comments and feedback, Dave. And I'm sorry
for the confusion.
Thank you!
--
Gustavo
[1] https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/