Re: Universally available bpf_ctx WAS: Re: [PATCH v2 bpf-next 02/15] bpf: net: Avoid sk_setsockopt() taking sk lock when called from bpf

On Thu, Aug 4, 2022 at 1:51 PM Andrii Nakryiko
[off-list ref] wrote:

On Thu, Aug 4, 2022 at 12:29 PM Martin KaFai Lau [off-list ref] wrote:

quoted

On Thu, Aug 04, 2022 at 12:03:04PM -0700, Andrii Nakryiko wrote:

quoted

On Wed, Aug 3, 2022 at 1:49 PM Martin KaFai Lau [off-list ref] wrote:

quoted

Most of the code in bpf_setsockopt(SOL_SOCKET) are duplicated from
the sk_setsockopt().  The number of supported optnames are
increasing ever and so as the duplicated code.

One issue in reusing sk_setsockopt() is that the bpf prog
has already acquired the sk lock.  This patch adds a in_bpf()
to tell if the sk_setsockopt() is called from a bpf prog.
The bpf prog calling bpf_setsockopt() is either running in_task()
or in_serving_softirq().  Both cases have the current->bpf_ctx
initialized.  Thus, the in_bpf() only needs to test !!current->bpf_ctx.

This patch also adds sockopt_{lock,release}_sock() helpers
for sk_setsockopt() to use.  These helpers will test in_bpf()
before acquiring/releasing the lock.  They are in EXPORT_SYMBOL
for the ipv6 module to use in a latter patch.

Note on the change in sock_setbindtodevice().  sockopt_lock_sock()
is done in sock_setbindtodevice() instead of doing the lock_sock
in sock_bindtoindex(..., lock_sk = true).

Signed-off-by: Martin KaFai Lau <redacted>
---
 include/linux/bpf.h |  8 ++++++++
 include/net/sock.h  |  3 +++
 net/core/sock.c     | 26 +++++++++++++++++++++++---
 3 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 20c26aed7896..b905b1b34fe4 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1966,6 +1966,10 @@ static inline bool unprivileged_ebpf_enabled(void)
        return !sysctl_unprivileged_bpf_disabled;
 }

+static inline bool in_bpf(void)

I think this function deserves a big comment explaining that it's not
100% accurate, as not every BPF program type sets bpf_ctx. As it is
named in_bpf() promises a lot more generality than it actually
provides.

Should this be named either more specific has_current_bpf_ctx() maybe?

Stans also made a similar point on this to add comment.
Rename makes sense until all bpf prog has bpf_ctx.  in_bpf() was
just the name it was used in the v1 discussion for the setsockopt
context.

quoted

Also, separately, should be make an effort to set bpf_ctx for all
program types (instead or in addition to the above)?

I would prefer to separate this as a separate effort.  This set is
getting pretty long and the bpf_getsockopt() is still not posted.

Yeah, sure, I don't think you should be blocked on that.

quoted

If you prefer this must be done first, I can do that also.

I wanted to bring this up for discussion. I find bpf_ctx a very useful
construct, if we had it available universally we could use it
(reliably) for this in_bpf() check, we could also have a sleepable vs
non-sleepable flag stored in such context and thus avoid all the
special handling we have for providing different gfp flags, etc.

+1

But it's not just up for me to decide if we want to add it for all
program types (e.g., I wouldn't be surprised if I got push back adding
this to XDP). Most program types I normally use already have bpf_ctx
(and bpf_cookie built on top), but I was wondering what others feel
regarding making this (bpf_ctx in general, bpf_cookie in particular)
universally available.

If we can get universal bpf_ctx, do we still need bpf_prog_active?
Regarding xdp: assigning a bunch of pointers shouldn't hopefully be
that big of a deal?