RE: [PATCH] openat2: switch to __attribute__((packed)) for open_how

From:  Aleksa Sarai

Sent: 15 December 2019 12:35
On 2019-12-14, Rasmus Villemoes [off-list ref] wrote:

quoted

On 13/12/2019 23.23, Aleksa Sarai wrote:

quoted

The design of the original open_how struct layout was such that it
ensured that there would be no un-labelled (and thus potentially
non-zero) padding to avoid issues with struct expansion, as well as
providing a uniform representation on all architectures (to avoid
complications with OPEN_HOW_SIZE versioning).

However, there were a few other desirable features which were not
fulfilled by the previous struct layout:

 * Adding new features (other than new flags) should always result in
   the struct getting larger. However, by including a padding field, it
   was possible for new fields to be added without expanding the
   structure. This would somewhat complicate version-number based
   checking of feature support.

 * A non-zero bit in __padding yielded -EINVAL when it should arguably
   have been -E2BIG (because the padding bits are effectively
   yet-to-be-used fields). However, the semantics are not entirely clear
   because userspace may expect -E2BIG to only signify that the
   structure is too big. It's much simpler to just provide the guarantee
   that new fields will always result in a struct size increase, and
   -E2BIG indicates you're using a field that's too recent for an older
   kernel.

And when the first extension adds another u64 field, that padding has to
be added back in and checked for being 0, at which point the padding is
again yet-to-be-used fields.

Maybe I'm missing something, but what is the issue with

  struct open_how {
    u64 flags;
    u64 resolve;
    u16 mode;
	u64 next_extension;
  } __attribute__((packed));

Compile anything that accesses it for (say) sparc and look at the object code.
You really, really, REALLY, don't want to EVER use 'packed'.

Just use u64 for all the fields.
Use 'flags' bits to indicate whether the additional fields should be looked at.
Error if a 'flags' bit requires a value that isn't passed in the structure.

Then you can add an extra field and old source code recompiled with the
new headers will still work - because the 'junk' value isn't looked at.

	David

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