[PATCH 1/5] lib/vsprintf.c: make sure vsnprintf() never returns a negative value
Sean Anderson
sean.anderson at seco.com
Fri May 21 16:48:29 CEST 2021
On 5/21/21 10:15 AM, 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."
But is this an encoding error? And of course, the most common use of
this function which checks this return value will be
n = vsnprintf(NULL, 0, fmt, ...);
buf = malloc(n);
if (!buf)
/* out of memory */
vsnprintf(buf, n, fmt, ...);
Which effectively already checks for ENOMEM. While it is
standard-compliant to return negative, it is also compliant to just use
a bogus value or to write nothing. There are many callers of
[v]s[c]nprintf which use the same pattern as the code above. If you
would like to fix all of them, go ahead. But I think this fix is more
efficient a cut.
--Sean
>
> 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.
>>
>> Rasmus
>>
>
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