Re: [PATCH v8 0/4] Introduce mseal

[Jeff Xu]

On Fri, Feb 2, 2024 at 12:37 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 2 Feb 2024 at 11:32, Theo de Raadt <deraadt@xxxxxxxxxxx> wrote:
> >
> > Unix system calls must be atomic.
> >
> > They either return an error, and that is a promise they made no changes.
>
> That's actually not true, and never has been.
>
> It's a good thing to aim for, but several errors means "some or all
> may have been done".
>
> EFAULT (for various system calls), ENOMEM and other errors are all
> things that can happen after some of the system call has already been
> done, and the rest failed.
>
> There are lots of examples, but to pick one obvious VM example,
> something like mlock() may well return an error after the area has
> been successfully locked, but then the population of said pages failed
> for some reason.
>
> Of course, implementations can differ, and POSIX sometimes has insane
> language that is actively incorrect.
>
> Furthermore, the definition of "atomic" is unclear. For example, POSIX
> claims that a "write()" system call is one atomic thing for regular
> files, and some people think that means that you see all or nothing.
> That's simply not true, and you'll see the write progress in various
> indirect ways (look at intermediate file size with 'stat', look at
> intermediate contents with 'mmap' etc etc).
>
> So I agree that atomicity is something that people should always
> *strive* for, but it's not some kind of final truth or absolute
> requirement.
>
> In the specific case of mseal(), I suspect there are very few reasons
> ever *not* to be atomic, so in this particular context atomicity is
> likely always something that should be guaranteed. But I just wanted
> to point out that it's most definitely not a black-and-white issue in
> the general case.
>
Thanks.
At least I got this part done right for mseal() :-)

-Jeff


>              Linus
>