What is the mincore syscall used for in userland applications?

Question

What is Linux' mincore(2) useful for in userland applications? Why is it exposed to non-privileged users?

I can imagine some databases taking advantage of knowing which pages are cached but what are some other examples?

Answer 1

What is Linux mincore(2) useful for in userland applications?

I'd say that's most probably opinion based. Profiling, statistics, performance evaluation and stuff like that comes to mind. Other than that, I cannot think of other realistic legitimate use cases.

Here are some examples of programs I found that use mincore (as you can see, all profiling/statistics related):

• http://man7.org/linux/man-pages/man1/fincore.1.html
• https://github.com/fornwall/pagecache
• https://github.com/bwaldvogel/mongocachestat
• https://github.com/touzaniMarouane/BlackLab

Why is it exposed to non-privileged users?

This would have actually been a good question around one year ago , when the syscall's semantics weren't clearly defined, and the existence of such a syscall was rather questionable. Prior to kernel version 4.14.2 mincore could in fact have been abused to leak uninitialized kernel memory from user space (see CVE-2017-16994 and the relative Project Zero bug report ).

Since then the syscall has been patched and its semantics updated. The only thing that a process can do by invoking it is to query information about its virtual memory map. Nothing harmful really, just self-inspection, hence the availability to unprivileged processes. There's no real reason to make it a privileged syscall, which if done could be also considered an API breakage.