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

Heinrich Schuchardt xypron.glpk at gmx.de
Fri May 21 16:42:04 CEST 2021

On 21.05.21 16:27, Tom Rini wrote:
> On Fri, May 21, 2021 at 04:15:39PM +0200, Heinrich Schuchardt wrote:
>> On 21.05.21 14:53, Rasmus Villemoes wrote:
>>> 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.
>> include/vsnprintf.h states:
>> "This function follows C99 vsnprintf, but has some extensions:".
>> The C99 spec says:
>> "The vsnprintf function returns the number of characters that would have
>> been written had n been sufficiently large, not counting  the
>> terminating  null  character, or a negative value if an encoding error
>> occurred."
>> It is obvious that the calling code needs to be fixed if it cannot
>> handle negative return values.
>> So NAK to the patch.
>> Best regards
>> Heinrich
>>> 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.
> So, how can we fix the callers without the above noted problems?

The assumption that vsnprintf() is used to print to the console and that
writing some arbitrary string to the buffer is allowable is utterly wrong.

vsnprintf_internal() is used to implement snprintf(). snprintf() is used
in numerous places where it will not lead to console output.

Trying to solve one problem this patch creates a bunch of new ones.

Best regards


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