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Re: [PATCH] Implement strlcpy, strlcat [BZ #178]

On 05/20/2016 07:21 PM, Paul Eggert wrote:
Two thoughts.

First, the September 2014 thread motivated strlcpy/strlcat in part by
saying that Fedora has over 60 packages which define strlcpy/strlcat[1],
the implication being that moving strlcpy/strlcat into glibc would fix
more bugs than it would cause and would be worth the effort's cost.
Skeptics like me can reasonably counter that the effort would likely
just be security theater, in that it would not be of any real help but
would merely exhibit activity in the security area, and possibly would
even cause more problems than it fixes.

I'm not sure if it is possible to easily quantify the security benefits that strlcpy/strlcat bring. Any project we start in this area would prevent us from working directly on glibc improvements, due to the resource constraints we face.

Back in 2014, I at least wasn't aware how widely strlcpy had been adopted by libcs. Since then, I have done some research, and all Linux libcs I know of except glibc provide it, but it's available well beyond that:

The major omissions are glibc and MSVC. Most of the other libcs in the ânoâ camp are legacy implementations to various degrees.

The compatibility doubts raised in 2015/earlier this year do not appear as significant as we assumed.

Data could help assuage skeptics' concerns. Although it's impossible to
count bugs that are not discovered yet, we *do* now have twenty months'
worth of data since September 2014. So: how many user-visible bugs
and/or vulnerabilities have been discovered or reported in these 60-odd
Fedora packages, bugs that would have been prevented by moving
strlcpy/strlcat into a better-maintained library?

At present, Fedora deliberately does not use libbsd widely (we eliminated the dependency from the Samba packages, for instance). As a result, the data you are asking for is scattered across the entire distribution. I don't know a good way to obtain it.

Second, the September 2014 thread said that another way to fortify
strlcpy/strlcat, assuming we still want to do it, would be to use
libbsd's strlcpy/strlcat implementation and add fortification to it.
However, it was argued that this would be a Fedora-only effort and that
libbsd contains functions like fgetln that are awkward. But isn't the
point of libbsd to provide BSD functions that are awkward or
questionable from the glibc point of view?

fgetln is not questionable at all by itself (except when reading lines from a read-only mapping or in-memory source, so that the non-const return value is incorrect). Its awkwardness is the result of the implementation in libbsd, on top of stdio interface (and not libio internals).

Once we have a clear picture of the vtable/ABI situation of libio, we can implement fgetln in glibc quite easily (with the caveat that writing to the returned buffer may not be allowed, depending on the underlying stream, but the BSDs have the same problem). But that's a separate discussion.

And I don't see how work in
libbsd would be Fedora-only, as other distributions like Debian also use
libbsd, and whatever methods Fedora uses to merge strlcpy/strlcat into
string.h would be methods that other distributions could use as well.

I might be misremembering things, but I think back in 2014, libbsd maintainership in Debian languished a bit, and the upstream situation was rather unclear.

The Fedora bug report that corresponds to strlcpy/strlcat
fortification[2] says that this is "Priority low Severity low", and from
the above it appears that this is mainly a packaging problem instead of
being a necessary addition to the core libc API.

We did not invest in libbsd development because the right place to offer widely-implemented libc APIs in Fedora is glibc.


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