On Sun, Sep 06, 2020 at 10:23:40PM -0400, Jeff King wrote: > On Sun, Sep 06, 2020 at 03:02:35PM -0400, Taylor Blau wrote: > > > On Sun, Sep 06, 2020 at 10:59:06AM +0200, René Scharfe wrote: > > > - uint32_t hash_value = htonl(entry->hash); > > > - hashwrite(f, &hash_value, sizeof(hash_value)); > > > + hashwrite_be32(f, entry->hash); > > > > This is an obviously correct translation of what's already written, and > > indeed it is shorter and easier to read. > > > > Unfortunately, I think there is some more subtlety here since the hash > > cache isn't guarenteed to be aligned, and so blindly calling htonl() > > (either directly in write_hash_cache(), or indirectly in > > hashwrite_be32()) might cause tools like ASan to complain when loading > > data on architectures that don't support fast unaligned reads. > > I think the alignment here is fine. We're just writing out an individual > value. So in the original entry->hash and our local hash_value are both > properly aligned, since they're declared as uint32_t. We pass the > pointer to hashwrite(), but it doesn't expect any particular alignment. > After the patch, the situation is the same, except that we're working > with the uint32_t parameter to hashwrite_be32(), which is also properly > aligned. Ack; I would blame it on skimming the patch, but this is far too obvious for that. The bug is on the *reading* end in GitHub's fork, and in a (custom) extension (which it looks like you describe below). Embarrassing. > > So, I think that we could do one of three things, depending on how much > > you care about improving this case ;-). > > > > - leave your patch alone, accepting that this case which was broken > > before will remain broken, and leave it as #leftoverbits > > So I think this is what we should do. :) Yep, this patch is correct as-is. > > - change the 'hashwrite_beXX()' implementations to use the correct > > 'get_beXX' wrappers which behave like htonl() on architectures with > > fast unaligned loads, and fall back to byte reads and shifts on > > architectures that don't. > > Likewise, I don't think there's any reason to do this. hashwrite_be32() > gets its parameter as a value, not a pointer. So even if it were coming > from an unaligned mmap, it's actually the _caller_ who would have to > use get_be32() when passing it. Right. > > Credit goes to Peff for finding this issue in GitHub's fork. For what > > it's worth, we were planning on sending those patches to the list soon, > > but they are tied up with a longer series in the meantime. > > There is a bug in our fork, but I don't think it's upstream. [...] Agreed with all of that, too. Taylor
