On Wed, Jan 25, 2023 at 10:37 AM Matthew Wilcox willy@infradead.org wrote:
On Wed, Jan 25, 2023 at 08:49:50AM -0800, Suren Baghdasaryan wrote:
On Wed, Jan 25, 2023 at 1:10 AM Peter Zijlstra peterz@infradead.org wrote:
/** Flags, see mm.h.* WARNING! Do not modify directly.* Use {init|reset|set|clear|mod}_vm_flags() functions instead.*/unsigned long vm_flags;We have __private and ACCESS_PRIVATE() to help with enforcing this.
Thanks for pointing this out, Peter! I guess for that I'll need to convert all read accesses and provide get_vm_flags() too? That will cause some additional churt (a quick search shows 801 hits over 248 files) but maybe it's worth it? I think Michal suggested that too in another patch. Should I do that while we are at it?
Here's a trick I saw somewhere in the VFS:
union { const vm_flags_t vm_flags; vm_flags_t __private __vm_flags; };Now it can be read by anybody but written only by those using ACCESS_PRIVATE.
Huh, this is quite nice! I think it does not save us from the cases when vma->vm_flags is passed by a reference and modified indirectly, like in ksm_madvise()? Though maybe such usecases are so rare (I found only 2 cases) that we can ignore this?