On 09.05.24 13:05, Vincent Donnefort wrote:
On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote:
On Tue, 30 Apr 2024 12:13:51 +0100
Vincent Donnefort <vdonnef...@google.com> wrote:
+#ifdef CONFIG_MMU
+static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
+ struct vm_area_struct *vma)
+{
+ unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
+ unsigned int subbuf_pages, subbuf_order;
+ struct page **pages;
+ int p = 0, s = 0;
+ int err;
+
+ /* Refuse MP_PRIVATE or writable mappings */
+ if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
+ !(vma->vm_flags & VM_MAYSHARE))
+ return -EPERM;
+
+ /*
+ * Make sure the mapping cannot become writable later. Also tell the VM
+ * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND). Finally,
+ * prevent migration, GUP and dump (VM_IO).
+ */
+ vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
Do we really need the VM_IO?
When testing this in gdb, I would get:
(gdb) p tmap->map->subbuf_size
Cannot access memory at address 0x7ffff7fc2008
It appears that you can't ptrace IO memory. When I removed that flag,
gdb has no problem reading that memory.
Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations.
Yes, the VM should recognize that memory to some degree as being special
already due to VM_MIXEDMAP and VM_DONTEXPAND.
#define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)
So any of these flag achieve that (e.g., mlock_fixup() checks
VM_SPECIAL). KSM similarly skips VM_DONTEXPAND and VM_MIXEDMAP (likely
we should be using VM_SPECIAL in vma_ksm_compatible()). Not sure about
page migration, likely its fine.
Thinking about MADV_DONTNEED, I can spot in
madvise_dontneed_free_valid_vma() only that we disallow primarily VM_PFNMAP.
... I assume if user space MADV_DONTNEED's some pages we'll simply get a
page fault later on access that will SIGBUS, handling that gracefully
(we should double-check!).
But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are
enough protection?
Do we want to dump these pages? VM_DONTDUMP might be reasonabe then.
I don't see how anything could use GUP there and as David pointed-out on the
previous version, it doesn't event prevent the GUP-fast path.
Yes, GUP-fast would still have worked under some conditions.
--
Cheers,
David / dhildenb
