fix(driver/kmod): always send fds to userspace in poll/ppoll syscall exit

[mrgian]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area API-version

/area build

/area CI

/area driver-kmod

/area driver-bpf

/area driver-modern-bpf

/area libscap-engine-bpf

/area libscap-engine-gvisor

/area libscap-engine-kmod

/area libscap-engine-modern-bpf

/area libscap-engine-nodriver

/area libscap-engine-noop

/area libscap-engine-source-plugin

/area libscap-engine-savefile

/area libscap

/area libpman

/area libsinsp

/area tests

/area proposals

Does this PR require a change in the driver versions?

/version driver-API-version-major

/version driver-API-version-minor

/version driver-API-version-patch

/version driver-SCHEMA-version-major

/version driver-SCHEMA-version-minor

/version driver-SCHEMA-version-patch

What this PR does / why we need it:
This uniforms the kmod driver's behavior to that of bpf and mbpf drivers in poll/ppoll syscall

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[github-actions]

Please double check driver/API_VERSION file. See versioning.

/hold

[FedeDP]

/approve

[poiana]

LGTM label has been added.

Git tree hash: 1c6fa2e20775c864d01a935515b211fdaec51de2

[FedeDP]

More standardization across the drivers is always welcome, thanks!

[FedeDP]

Me and @therealbobo were already investigating the failing test:

[ FAILED ] SyscallExit.process_vm_readvX_failure

You should probably modify that test by changing
size_t res = syscall(__NR_process_vm_readv, getpid(), iov, iovcnt, iov, iovcnt, 0);
to
size_t res = syscall(__NR_process_vm_readv, getpid(), iov, iovcnt, iov, iovcnt, 1);
ie: passing a non-empty flag.

This will trigger:

 EINVAL flags is not 0.

EDIT: anway, i'd love to make it fail in a different way since sooner or later the flags != 0 check might go away (ie: when flags will actually be an used parameter).

[FedeDP]

/approve

[poiana]

LGTM label has been added.

Git tree hash: df781de9f9b54071370e45459713867d5598f0ba

[mrgian]

Looks like SyscallEnter.spliceE is failing too 🤔

[FedeDP]

/approve

[poiana]

LGTM label has been added.

Git tree hash: 2d05938cb604d0dcb4c05ffb5163ca1d583853aa

[FedeDP]

/milestone next-driver

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, FedeDP, mrgian

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[FedeDP]

/unhold