On Wed, Sep 13, 2017 at 10:53:57AM -0700, Jonathan Nieder wrote:
> Jeff King wrote:
>
> > We ask to write 41 bytes and make sure that the return value
> > is at least 41. This is the same "dangerous" pattern that
> > was fixed in the prior commit (wherein a negative return
> > value is promoted to unsigned), though it is not dangerous
> > here because our "41" is a constant, not an unsigned
> > variable.
> >
> > But we should convert it anyway to avoid modeling a
> > dangerous construct.
>
> If the above logic is correct, then I suspect this series does not go
> far enough. write_in_full() would be one of those APIs that is easy
> to misuse and difficult to use correctly, and if so we should fix that
> at the source instead of trying to teach callers not to hold it wrong.
Yes, this series is just removing bad examples. It doesn't do anything
to make write_in_full() less potentially dangerous.
On the other hand, it's no more or less dangerous than write(), which
has the same return-value semantics.
> E.g. what would you think of
>
> 1. Introduce a write_fully (sorry, I am bad at names) function
> that returns 0 on success and a coccinelle semantic patch in
> contrib/coccinelle to migrate callers in "make coccicheck":
Yes, I considered that, though some callers really do care about
assigning the number of bytes written. The fact that write() has the
same problem, plus the fact that there were only 2 buggy instances
across the code base made me think there's not a huge gain to that extra
step.
> @@
> expression E;
> expression F;
> expression G;
> @@
> -write_in_full(E, F, G) < G
> +write_fully(E, F, G)
>
> 2. Run "make coccicheck" and apply the result.
> 3. Remove the write_in_full function.
There's a step between those where you have to update all of the
write_in_full() callers that store the result. Some of them would be
trivial conversions, but some of them actually care about the length
E.g., the one in imap-send.c, which is the only one I didn't convert
away from "!= len" because it's half of an #ifdef with SSL_write()
(which uses an "int" as the return value!).
> Does read_in_full need a similar treatment?
It might actually return fewer than the requested number of bytes, so it
can't just use "< 0" in the same way (nor be adapted to return 0 on
success). But I think it's still a bug to do:
char buf[20];
size_t len = sizeof(buf);
if (read_in_full(fd, buf, len) < len)
die(...);
since that will promote the -1 to a size_t. So it's probably worth
auditing.
-Peff
