[tz] Minor (unimportant really) technical UB bug in strftime() ?

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Sat Nov 12 00:13:07 UTC 2022

On 2022-11-11 13:53, Paul Eggert via tz wrote:
> On 2022-11-10 16:11, Tom Lane wrote:
>> simple modifications of the
>> strftime() case can provide pretty good examples.
> I'm not sure I'm following this argument, but if I understand it correctly I 
> disagree. I very much much want implementations to reject attempts to use 
> uninitialized data, and I don't want the C standard to prohibit such 
> implementations. Use of uninitialized data is a serious problem in real 
> applications - several CVEs have arisen from it - and the C standard shouldn't 
> require implementations to sweep it under the rug.

> This call is obviously buggy since it is passing an uninitialized struct tm to 
> strftime. However, the C standard says the patched strftime's memcpy initializes 
> strftime's local variable tm, and therefore mktime's use of tm's contents is 
> defined and the implementation allow the program's use of uninitialized data.
> Whatever the reason for this peculiarity of the C standard (and nobody seems to 
> remember why it's there), it's counterproductive. In general C programs 
> shouldn't use uninitialized data. If they do, it's useful to have a debugging 
> implementation that reports any such use the instant that it happens as this 
> will help catch real bugs. And since common debugging implementations like 
> valgrind and UBSan are not following the C standard here, this suggests that the 
> C standard is mistaken, not the implementations.

>> That leads either to strftime() knowing
>> much more about mktime()'s innards than it ought to

> It's not a problem for strftime, since strftime's has to know what's needed in 
> the struct tm; this follows from strftime's API.

It may not know everything about the object, as there may be implementatiion 
extensions to deal with e.g. localization issues, and can never know everything 
about another function implementation, which could be changed at any time.

> It might be a problem for other scenarios, such as the one you vaguely suggested 
> though I don't know the details. However, even assuming these other scenarios 
> exist, they don't justify this peculiarity of the C standard. The standard 
> should not say that the ordinary meaning of "don't use uninitialized variables" 
> is wrong and that there's some trickier interpretation that hardly any 
> programmer knows and that was wrong even in C17 and so needed rewording (and 
> perhaps still needs rewording) and that nevertheless must be supported - even 
> though important debugging implementations disagree.

I think the principle is just to program carefully and generically around 
possible issues, when dealing with an opaque object you do not define and 
control, to avoid creating issues with private fields or functions which may be 
part of the implementation, but not in whatever documented interface you are 
programming to.

Not all the world is...C, POSIX, or Linux, compliant.

Take care. Thanks, Brian Inglis			Calgary, Alberta, Canada

La perfection est atteinte			Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter	not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer	but when there is no more to cut
			-- Antoine de Saint-Exupéry

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