ebpf: Call bpf_probe_read on *const T BTF arguments

[vadorovsky]

It's necessary to call bpf_probe_read not only for pointers retrieved from PtRegs, but also from BTF arguments.

bpf_probe_read might return an error, so the return type of .arg() methods in contexts handling BTF arguments changes from Self to Option<Self>. None is returned when bpf_probe_read call is not successful.

Fixes: #542

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[tamird]

@vadorovsky mind rebasing?

[mergify]

@vadorovsky, this pull request is now in conflict and requires a rebase.

[mergify]

@vadorovsky, this pull request is now in conflict and requires a rebase.

[kamyuentse]

LGTM. I think this should be merged. @vadorovsky mind rebasing?

[tamird]

I rebased this, but I think the tests need more work.

[tamird]

Reviewed 7 of 9 files at r1, 6 of 6 files at r2, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @vadorovsky)

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test/integration-test/src/tests/args.rs line 1 at r2 (raw file):

use aya::{

not clear to me what we're testing here

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test/integration-ebpf/src/args.rs line 11 at r2 (raw file):

#[kprobe]
pub fn kprobe_vfs_write(ctx: ProbeContext) {
    let _: Option<usize> = ctx.arg(3);

what exactly are we testing here?

[tamird]

Reviewed 5 of 5 files at r3, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @vadorovsky)