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

Linus Torvalds
2021-08-14T00:40:05Z

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.

Linus