[PATCH 1/5] lib/vsprintf.c: make sure vsnprintf() never returns a negative value

[Rasmus Villemoes]

On 20/05/2021 19.51, Simon Glass wrote:
> Hi Rasmus,
> 
> On Thu, 20 May 2021 at 04:05, Rasmus Villemoes
> <rasmus.villemoes at prevas.dk> wrote:
>>
>> Most callers (or callers of callers, etc.) of vsnprintf() are not
>> prepared for it to return a negative value.
>>
>> The only case where that can currently happen is %pD, and it's IMO
>> more user-friendly to produce some output that clearly shows that some
>> "impossible" thing happened instead of having the message completely
>> ignored - or mishandled as for example log.c would currently do.
>>
>> Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
>> ---
>>  lib/vsprintf.c | 10 +---------
>>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> I think that is debatable. If we want the calling code to be fixed,
> then it needs to get an error code back. Otherwise the error will be
> apparent to the user but (perhaps) not ever debugged.

But it is not the calling code that is at fault for the vsnprintf()
implementation (1) being able to fail and (2) actually encountering an
ENOMEM situation. There's _nothing_ the calling code can do about that.

The calling code can be said to be responsible for not passing NULL
pointers, but that case is actually handled gracefully in various places
in the printf code (both for %pD, but also plain %s).

> The definition of printf() allows for the possibility of a negative
> return value.

First, please distinguish printf() from vsnprintf(). The former (in the
normal userspace version) obviously can fail for the obvious EIO, ENOSPC
reasons. The latter is indeed allowed to fail per the posix spec, but
from a QoI perspective, I'd say it's much better to have a guarantee
_for our particular implementation_ that it does not fail (meaning:
returns a negative result). There's simply too many direct and indirect
users of vsnprintf() that assume the result is non-negative; if we do
not provide that guarantee, the alternative is to play a whack-a-mole
game and add tons of error-checking code (adding bloat to the image),
with almost never any good way to handle it.

Take that log_info(" ... %pD") as an example. Suppose we "fix" log.c so
that it ignores the message if vsnprintf (or vscnprintf, whatever)
returns a negative result, just as print() currently does [which is the
other thing that log_info could end up being handled by]. That means
nothing gets printed on the console, and nobody gets told about the
ENOMEM. In contrast, with this patch, we get

  Booting <%pD:ENOMEM>

printed on the console, so at least _some_ part of the message gets out,
and it's apparent that something odd happened. Of course, all of that is
in the entirely unlikely sitation where the (efi) allocation would
actually fail.

If we don't want that <%pD:ENOMEM> thing, I'd still argue that we should
ensure vsnprintf returns non-negative; e.g. by changing the "return
PTR_ERR()" to a "goto out", i.e. simply stop the processing of the
format string at the %pD which failed, but still go through the epilogue
that ensures the resulting string becomes nul-terminated (another
reasonable assumption made by tons of callers), and return how much got
printed till then.

Rasmus