On Fri, Jan 17, 2020 at 4:07 PM Alexei Starovoitov [off-list ref] wrote:

Introduce dynamic program extensions. The users can load additional BPF
functions and replace global functions in previously loaded BPF programs while
these programs are executing.

Global functions are verified individually by the verifier based on their types only.
Hence the global function in the new program which types match older function can
safely replace that corresponding function.

This new function/program is called 'an extension' of old program. At load time
the verifier uses (attach_prog_fd, attach_btf_id) pair to identify the function
to be replaced. The BPF program type is derived from the target program into
extension program. Technically bpf_verifier_ops is copied from target program.
The BPF_PROG_TYPE_EXT program type is a placeholder. It has empty verifier_ops.
The extension program can call the same bpf helper functions as target program.
Single BPF_PROG_TYPE_EXT type is used to extend XDP, SKB and all other program
types. The verifier allows only one level of replacement. Meaning that the
extension program cannot recursively extend an extension. That also means that
the maximum stack size is increasing from 512 to 1024 bytes and maximum
function nesting level from 8 to 16. The programs don't always consume that
much. The stack usage is determined by the number of on-stack variables used by
the program. The verifier could have enforced 512 limit for combined original
plus extension program, but it makes for difficult user experience. The main
use case for extensions is to provide generic mechanism to plug external
programs into policy program or function call chaining.

BPF trampoline is used to track both fentry/fexit and program extensions
because both are using the same nop slot at the beginning of every BPF
function. Attaching fentry/fexit to a function that was replaced is not
allowed. The opposite is true as well. Replacing a function that currently
being analyzed with fentry/fexit is not allowed. The executable page allocated
by BPF trampoline is not used by program extensions. This inefficiency will be
optimized in future patches.

Function by function verification of global function supports scalars and
pointer to context only. Hence program extensions are supported for such class
of global functions only. In the future the verifier will be extended with
support to pointers to structures, arrays with sizes, etc.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 include/linux/bpf.h       |  10 ++-
 include/linux/bpf_types.h |   2 +
 include/linux/btf.h       |   5 ++
 include/uapi/linux/bpf.h  |   1 +
 kernel/bpf/btf.c          | 152 +++++++++++++++++++++++++++++++++++++-
 kernel/bpf/syscall.c      |  15 +++-
 kernel/bpf/trampoline.c   |  38 +++++++++-
 kernel/bpf/verifier.c     |  84 ++++++++++++++++-----
 8 files changed, 281 insertions(+), 26 deletions(-)

[...]

move this check before BPF_TRAMP_REPLACE check and check additonally
for fentry+fexit for BPF_TRAMP_REPLACE? Nothing can replace
extension_prog, right?

        if (tr->progs_cnt[BPF_TRAMP_FENTRY] + tr->progs_cnt[BPF_TRAMP_FEXIT]
            >= BPF_MAX_TRAMP_PROGS) {
                err = -E2BIG;

[...]

if the intent is to prevent extending FENTRY/FEXIT, why not checking
explicitly for those two instead of making assumption that
expected_attach_type can be only one of RAW_TP/FENTRY/FEXIT, this can
easily change in the future. Besides, direct FENTRY/FEXIT comparison
is more self-documenting as well.

+                   prog_extension) {
+                       /* Program extensions can extend all program types
+                        * except fentry/fexit. The reason is the following.
+                        * The fentry/fexit programs are used for performance
+                        * analysis, stats and can be attached to any program
+                        * type except themselves. When extension program is
+                        * replacing XDP function it is necessary to allow
+                        * performance analysis of all functions. Both original
+                        * XDP program and its program extension. Hence
+                        * attaching fentry/fexit to BPF_PROG_TYPE_EXT is
+                        * allowed. If extending of fentry/fexit was allowed it
+                        * would be possible to create long call chain
+                        * fentry->extension->fentry->extension beyond
+                        * reasonable stack size. Hence extending fentry is not
+                        * allowed.
+                        */
+                       verbose(env, "Cannot extend fentry/fexit\n");
+                       return -EINVAL;
+               }
                key = ((u64)aux->id) << 32 | btf_id;

[...]

this reads so weird... btf_check_type_match (and
btf_check_func_type_match as well) are boolean functions (i.e., either
matches or not, or some error), why not using a conventional
boolean+error return convention: 0 - false, 1 - true, <0 - error
(bug)?

+                       return -EINVAL;
                t = btf_type_by_id(btf, t->type);
                if (!btf_type_is_func_proto(t))
                        return -EINVAL;

[...]