On 9/8/21 11:30 AM, Liam Howlett wrote:
* Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> [210908 14:21]:
On Wed, Sep 8, 2021 at 11:15 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
On Wed, 8 Sep 2021 11:02:58 -0700 Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
Please describe the expected userspace-visible change from Peter's
patch in full detail?
User space expects build_id to be available. Peter patch simply removes
that feature.
Are you sure? He ends up with
More than sure :)
Just look at below.
static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
u64 *ips, u32 trace_nr, bool user)
{
int i;
/* cannot access current->mm, fall back to ips */
for (i = 0; i < trace_nr; i++) {
id_offs[i].status = BPF_STACK_BUILD_ID_IP;
id_offs[i].ip = ips[i];
memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
}
return;
}
and you're saying that userspace won't like this because we didn't set
BPF_STACK_BUILD_ID_VALID?
The patch forces the "fallback path" that in production is seen 0.001%
Meaning that user space doesn't see build_id any more. It sees IPs only.
The user space cannot correlate IPs to binaries. That's what build_id enabled.
I was thinking of decomposing the checks in my first response to two
functions.
Something like this:
--------------
diff --git a/mm/mmap.c b/mm/mmap.c
index dce46105e3df..8afc1d22aa61 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2293,12 +2293,13 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
EXPORT_SYMBOL(get_unmapped_area);
/* Look up the first VMA which satisfies addr < vm_end, NULL if none. */
-struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
+struct vm_area_struct *find_vma_non_owner(struct mm_struct *mm,
+ unsigned long addr)
{
struct rb_node *rb_node;
struct vm_area_struct *vma;
- mmap_assert_locked(mm);
+ VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
/* Check the cache first. */
vma = vmacache_find(mm, addr);
if (likely(vma))
@@ -2325,6 +2326,11 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
return vma;
}
+struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
+{
+ lockdep_assert_held(&mm->mmap_lock);
+ return find_vma_non_owner(mm, addr);
+}
EXPORT_SYMBOL(find_vma);
/*
--------------
Although this leaks more into the mm API and was referred to as ugly
previously, it does provide a working solution and still maintains the
same level of checking.
Would it push the back actors to just switch to non_owner though?
Thanks, Liam. This should work for bpf side as well as we can just call
find_vma_no_owner(). I will submit v3 soon.
Thanks,
Liam
