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
