Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

Andy Lutomirski
2021-08-14T00:49:45Z

On Fri, Aug 13, 2021, at 5:31 PM, Linus Torvalds wrote:
> On Fri, Aug 13, 2021 at 10:18 AM Eric W. Biederman
> <ebiederm@xmission.com> wrote:
> >
> > Florian Weimer, would it be possible to get glibc's ld.so implementation to use
> > MAP_SHARED? Just so people reading the code know what to expect of the
> > kernel? As far as I can tell there is not a practical difference
> > between a read-only MAP_PRIVATE and a read-only MAP_SHARED.
>
> There's a huge difference.
>
> For one, you actually don't necessarily want read-only. Doing COW on
> library images is quite common for things like relocation etc (you'd
> _hope_ everything is PC-relative, but no)
>
> So no. Never EVER use MAP_SHARED unless you literally expect to have
> two different mappings that need to be kept in sync and one writes the
> other.
>
> I'll just repeat: stop arguing about this case. If somebody writes to
> a busy library, THAT IS A FUNDAMENTAL BUG, and nobody sane should care
> at all about it apart from the "you get what you deserve".
>
> What's next? Do you think glibc should also map every byte in the user
> address space so that user programs don't get SIGSEGV when they have
> wild pointers?
>
> Again - that's a user BUG and trying to "work around" a wild pointer
> is a worse fix than the problem it tries to fix.
>
> The exact same thing is true for shared library (or executable)
> mappings. Trying to work around people writing to them is *worse* than
> the bug of doing so.
>
> Stop this completely inane discussion already.
>

I’ll bite. How about we attack this in the opposite direction: remove the deny write mechanism entirely.

In my life, I’ve encountered -ETXTBUSY intermittently, and it invariably means that I somehow failed to finish killing a program fast enough for whatever random rebuild I’m doing to succeed. It’s at best erratic — it only applies for static binaries, and it has never once saved me from a problem I care about. If the program I’m recompiling crashes, I don’t care — it’s probably already part way through dying from an unrelated fatal signal. What actually happens is that I see -ETXTBUSY, think “wait, this isn’t Windows, why are there file sharing rules,” then think “wait, Linux has *one* half baked file sharing rule,” and go on with my life. [0]

Seriously, can we deprecate and remove the whole thing?

[0] we have mandatory locks, too. Sigh.