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From: Pablo Correa Gomez <ablocorrea@hotmail.com>
To: Rich Felker <dalias@libc.org>
Cc: musl@lists.openwall.com
Subject: Re: [musl] newlocale: Segmentation fault when locale input is NULL
Date: Wed, 06 Oct 2021 14:57:55 +0200	[thread overview]
Message-ID: <AM5P192MB00811E790FEDDEAFD163D694C7B09@AM5P192MB0081.EURP192.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <20211006122936.GD2559@brightrain.aerifal.cx>

Thank you very much to both for the detailed explanation. I really
appreciate you took the time to explain why musl behaviour should
remain the way it is. I will proceed and fix the bug in GNOME.

> In particular, here it seems to have found a bug -- what could the
> application have possibly meant by passing a null pointer there? Did
> it actually intend the behavior of ""? Or of "C"? Or if the intent
> was
> to have this mean "don't use a context-local locale", why pass the
> pointer to newlocale and process the error (which could include a
> number of other errors you'd certainly want to treat differently)
> rather than checking for null and taking a different code path before
> calling newlocale?

Very valid point. Definitely something worth to ask the maintainers.

Best and thank you again,
Pablo.

On Wed, 2021-10-06 at 08:29 -0400, Rich Felker wrote:
> On Wed, Oct 06, 2021 at 11:31:29AM +0200, Pablo Correa Gomez wrote:
> > Dear musl maintainers,
> > 
> > While doing some work in GNOME control center for postmarketos, we
> > bumped into a segmentation fault which is also present in GNOME in
> > Alpine[1].
> > 
> > After doing some degugging, I figured out that the reason is that,
> > through GNOME desktop[2], there is a call to newlocale, where they
> > end
> > up calling it with a NULL argument.
> > 
> > newlocale(LC_CTYPE, NULL, (locale_t)0);
> > 
> > In this case, "name" is passed to __get_locale in
> > src/locale/newlocale.c:27 and then dereferenced in
> > src/locale/locale_map.c:43, causing a segmentation fault.
> > 
> > In the case of glibc, this is not an issue, as per the
> > documentation[3]
> > they consider it an error:
> > 
> >        EINVAL locale is NULL.
> > 
> > Unfortunately, this is a difference in the implementation between
> > glibc
> > and musl, maybe due to the fact that the standard[4] in not clear
> > in
> > this point:
> > 
> > 
> > The newlocale() function may fail if:
> > 
> > [EINVAL]
> >     The locale argument is not a valid string pointer. 
> 
> This is specifically documented as a "may fail", not a "shall fail",
> i.e. it's not guaranteed to happen. It comes from POSIX, and is an
> instance of a weird pattern the committee tried to fix (but missed
> some places it seems) where they wrote "may fail"s for conditions
> that
> already have undefined behavior (here, use of an invalid pointer) in
> which case EINVAL would already be allowed as a side effect of the UB
> without any further specification. (The same thing created a lot of
> confusion in the past about use of pthread_t values past the end of
> their lifetime.)
> 
> > My personal believe is that adding a NULL pointer check in musl is
> > very
> > simple and might help not only GNOME desktop, but maybe also other
> > projects in the future. This is the reason why I brought the issue
> > here
> > first instead of directly patching GNOME desktop. If you believe
> > that
> > musl behaviour should remain the way it is, please let me know and
> > I
> > will send MRs for upstream and Alpine's GNOME desktop. I am not
> > subscribed to the mailing list, so I would appreciate if I am CC'ed
> > in
> > any response.
> 
> The musl behavior should remain the way it is. My text on the
> rationale actually made it into the glibc wiki some years back:
> 
> https://sourceware.org/glibc/wiki/Style_and_Conventions#Invalid_pointers
> 
>     "The GNU C library considers it a QoI feature not to mask user
>     bugs by detecting invalid pointers and returning EINVAL (unless
>     the API is standardized and says it does that). If passing a bad
>     pointer has undefined behavior, it is far more useful in the long
>     run if it crashes quickly rather than diagnosing an error that is
>     probably ignored by the flaky caller.
> 
>     If you're going to check for NULL pointer arguments where you
> have
>     not entered into a contract to accept and interpret them, do so
>     with an assert, not a conditional error return. This way the bugs
>     in the caller will be immediately detected and can be fixed, and
>     it makes it easy to disable the overhead in production builds.
> The
>     assert can be valuable as code documentation. However, a segfault
>     from dereferencing the NULL pointer is just as effective for
>     debugging. If you return an error code to a caller which has
>     already proven itself buggy, the most likely result is that the
>     caller will ignore the error, and bad things will happen much
>     later down the line when the original cause of the error has
>     become difficult or impossible to track down. Why is it
> reasonable
>     to assume the caller will ignore the error you return? Because
> the
>     caller already ignored the error return of malloc or fopen or
> some
>     other library-specific allocation function which returned NULL to
>     indicate an error."
> 
> In particular, here it seems to have found a bug -- what could the
> application have possibly meant by passing a null pointer there? Did
> it actually intend the behavior of ""? Or of "C"? Or if the intent
> was
> to have this mean "don't use a context-local locale", why pass the
> pointer to newlocale and process the error (which could include a
> number of other errors you'd certainly want to treat differently)
> rather than checking for null and taking a different code path before
> calling newlocale?
> 
> Rich


  reply	other threads:[~2021-10-06 14:25 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-06  9:31 Pablo Correa Gomez
2021-10-06 12:29 ` Rich Felker
2021-10-06 12:57   ` Pablo Correa Gomez [this message]
2021-10-06 12:31 ` Quentin Rameau

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