linux: fix regression mounting within userns

[giuseppe]

commit 4a27212 introduced the regression.

Detect when running inside a user namespace and treat the mounts in the same way as they would be treated if creating a new user namespace.

Closes: #1855

Summary by Sourcery

Bug Fixes:

• Add a check for running inside an existing user namespace and set has_userns to enforce correct mount handling

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Introduces runtime detection of existing user namespaces when preparing mounts and applies the same mount handling logic as for newly created user namespaces to avoid regression.

Sequence diagram for mount preparation with user namespace detection

sequenceDiagram
    participant Container
    participant LinuxMountHandler
    participant UserNamespaceChecker
    Container->>LinuxMountHandler: prepare_and_send_mount_mounts()
    LinuxMountHandler->>UserNamespaceChecker: check_running_in_user_namespace()
    UserNamespaceChecker-->>LinuxMountHandler: is_in_userns (bool)
    LinuxMountHandler->>LinuxMountHandler: Set has_userns based on detection
    LinuxMountHandler->>LinuxMountHandler: Apply userns mount logic if has_userns
    LinuxMountHandler->>Container: Mounts prepared

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Class diagram for user namespace detection in mount preparation

classDiagram
    class LinuxMountHandler {
        +prepare_and_send_mount_mounts(container, pid, sys)
        -has_userns: bool
        -mount_fds
    }
    class UserNamespaceChecker {
        +check_running_in_user_namespace(err): int
    }
    LinuxMountHandler --> UserNamespaceChecker: uses
    LinuxMountHandler : has_userns set by check_running_in_user_namespace()
    LinuxMountHandler : applies userns mount logic if has_userns

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File-Level Changes

Change	Details	Files
Detect and treat mounts inside existing user namespaces like new user namespace mounts	Added a check for running in a user namespace when has_userns is false Called check_running_in_user_namespace and handled negative return as error Updated has_userns flag based on the detection result	src/libcrun/linux.c

Possibly linked issues

• Regression: rootless podman does not consider user supplementary group when mounting volume/file #1855: PR fixes crun's regression in user namespace mount handling, resolving issue 1855's failure.

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[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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[flouthoc]

LGTM