[PATCH 1/5] lib/vsprintf.c: make sure vsnprintf() never returns a negative value
xypron.glpk at gmx.de
Fri May 21 16:15:39 CEST 2021
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.
"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
It is obvious that the calling code needs to be fixed if it cannot
handle negative return values.
So NAK to the patch.
> 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.
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