Re: [PATCH v13 13/20] bpf, arm64: untag user pointers in stack_map_get_build_id_offset

Catalin Marinas
2019-03-22T15:52:27Z

On Wed, Mar 20, 2019 at 03:51:27PM +0100, Andrey Konovalov wrote:
> This patch is a part of a series that extends arm64 kernel ABI to allow to
> pass tagged user pointers (with the top byte set to something else other
> than 0x00) as syscall arguments.
>
> stack_map_get_build_id_offset() uses provided user pointers for vma
> lookups, which can only by done with untagged pointers.
>
> Untag user pointers in this function for doing the lookup and
> calculating the offset, but save as is in the bpf_stack_build_id
> struct.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
> kernel/bpf/stackmap.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 950ab2f28922..bb89341d3faf 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -320,7 +320,9 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
> }
>
> for (i = 0; i < trace_nr; i++) {
> - vma = find_vma(current->mm, ips[i]);
> + u64 untagged_ip = untagged_addr(ips[i]);
> +
> + vma = find_vma(current->mm, untagged_ip);
> if (!vma || stack_map_get_build_id(vma, id_offs[i].build_id)) {
> /* per entry fall back to ips */
> id_offs[i].status = BPF_STACK_BUILD_ID_IP;
> @@ -328,7 +330,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
> memset(id_offs[i].build_id, 0, BPF_BUILD_ID_SIZE);
> continue;
> }
> - id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ips[i]
> + id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + untagged_ip
> - vma->vm_start;
> id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
> }

Can the ips[*] here ever be tagged?

--
Catalin