On Tue, May 04, 2021 at 10:55:44AM -0700, harshad shirwadkar wrote: > > However, wouldn't it be easier to just add __attribute__((packed)) to the > > definition of struct journal_block_tag_t? > While we know that journal_block_tag_t can be unaligned, our code > should still ensure that we are reading this struct in an > alignment-safe way (like Ted's patch does). IIUC, using > __attribute__((packed)) might result in us keeping the door open for > unaligned accesses in future. If someone tries to read 4 bytes > starting at &journal_block_tag_t->t_flags, with attribute packed, > UBSAN won't complain but this may still cause issues on some > architectures. I don't understand your concern here. Accesses to a packed struct are assumed to be unaligned -- that's why I suggested it. The packed attribute is pretty widely used to implement unaligned accesses in C (as an alternative to memcpy() or explicit byte-by-byte accesses, both of which also work, though the latter seems to run into an UBSAN bug in this case). - Eric
