Re: Usage of "p" constraint in BPF inline asm

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

On Fri, 2023-08-11 at 16:10 +0200, Jose E. Marchesi wrote:
> > Do you need any help with the environment itself?
> > (I can describe my setup if you need that).
> 
> That would be useful yes, thank you.

There are several things needed:
- bpf-next source code:
  https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git
- Specific kernel configuration
- QEMU to run selftests in
- Root file system for QEMU to boot
- Means to kickstart tests execution inside the VM.

There is a script vmtest.sh located in the kernel repository:

    tools/testing/selftests/bpf/vmtest.sh
    
Which takes care of kernel configuration, compilation, selftests
compilation, qemu rootfs image download and test execution.

This is not exactly what I use but I tested in right now and it works
with a few caveats. Explaining my setup would take longer so I'll
start with this one. I will submit patches with fixes for caveats.

## Caveat #1: libc version

The script downloads rootfs image from predefined location on github
(aws?) and that image is based on debian bullseye. libc version on my
system is newer, so there is an error when test binaries built on my
system are executed inside VM. So, I have to prepare my own rootfs
image and point vmtest.sh to it. It might not be a problem in your
case, if so -- skip the rest of the section.

Unfortunately, there is no option to override rootfs via command line
of that script, so the following patch is needed:

```diff
diff --git a/tools/testing/selftests/bpf/vmtest.sh b/tools/testing/selftests/bpf/vmtest.sh
index 685034528018..3d0c7e7c0135 100755
--- a/tools/testing/selftests/bpf/vmtest.sh
+++ b/tools/testing/selftests/bpf/vmtest.sh
@@ -124,6 +124,15 @@ download_rootfs()
                exit 1
        fi
 
+        echo "download_rootfs: $ROOTFS_OVERRIDE"
+        if [[ "$ROOTFS_OVERRIDE" != "" ]]; then
+            if [[ ! -e $ROOTFS_OVERRIDE ]]; then
+               echo "Can't find rootfs image referred to by ROOTFS_OVERRIDE: $ROOTFS_OVERRIDE"
+               exit 1
+            fi
+            cat $ROOTFS_OVERRIDE | zstd -d | sudo tar -C "$dir" -x
+            exit
+        fi
        download "${ARCH}/libbpf-vmtest-rootfs-$rootfsversion.tar.zst" |
                zstd -d | sudo tar -C "$dir" -x
 }
```

Here is how to prepare the disk image for bookworm:

    $ git clone https://github.com/libbpf/ci libbpf-ci
    $ cd libbpf-ci
    $ sudo ./rootfs/mkrootfs_debian.sh -d bookworm
      # !! eddy -- is my user name locally, update accordingly
    $ sudo chown eddy libbpf-vmtest-rootfs-2023.08.11-bookworm-amd64.tar.zst
    $ export ROOTFS_OVERRIDE=$(realpath libbpf-vmtest-rootfs-2023.08.11-bookworm-amd64.tar.zst)

Script stores Qemu disk image in ~/.bpf_selftests/root.img .
We need to prepare/update that image using the following command:

      # !! make sure ROOTFS_OVERRIDE is set
    $ cd <kernel-sources>
    $ cd tools/testing/selftests/bpf
    $ ./vmtest.sh -i

(Note: script uses sudo internally, so it might ask for password).

## Caveat #2: make headers

Kernel compilation command requires the following patch:

```diff
diff --git a/tools/testing/selftests/bpf/vmtest.sh b/tools/testing/selftests/bpf/vmtest.sh
index 685034528018..3d0c7e7c0135 100755
--- a/tools/testing/selftests/bpf/vmtest.sh
+++ b/tools/testing/selftests/bpf/vmtest.sh
@@ -137,6 +146,7 @@ recompile_kernel()
 
        ${make_command} olddefconfig
        ${make_command}
+        ${make_command} headers
 }
 
 mount_image()
```

## Running tests

Running tests is simple:

    $ cd <kernel-sources>
    $ cd tools/testing/selftests/bpf
    $ ./vmtest.sh -- ./test_verifier

The script will rebuild both kernel and selftests if necessary.
The log should look as follows:

    $ ./vmtest.sh -- ./test_verifier
    Output directory: /home/eddy/.bpf_selftests
    ... build log ....
    [    0.000000] Linux version 6.5.0-rc4-g2adbb7637fd1-dirty ...
    ... boot log ...
    + /etc/rcS.d/S50-startup
    ./test_verifier
    #0/u BPF_ATOMIC_AND without fetch OK
    #0/p BPF_ATOMIC_AND without fetch OK
    #1/u BPF_ATOMIC_AND with fetch OK
    ... test_verifier log ...
    #524/p wide load from bpf_sock_addr.msg_src_ip6[3] OK
    Summary: 790 PASSED, 0 SKIPPED, 0 FAILED
    [    3.724015] ACPI: PM: Preparing to enter system sleep state S5
    [    3.725169] reboot: Power down
    Logs saved in /home/eddy/.bpf_selftests/bpf_selftests.2023-08-11_18-53-05.log

## Selecting individual tests

For test_verifier individual tests could be selected using command:

    $ ./vmtest.sh -- ./test_verifier -vv 42

(-vv forces detailed logging).

For test_progs/test_progs-no_alu32/test_progs-cpuv4 using the
following command:

    $ ./vmtest.sh -- ./test_progs-cpuv4 -vvv -a verifier_ldsx

(-a stands for allow and filters tests by names).

`test_maps` do not take any options AFAIK.

---

Hope this helps.
Feel free to ask about any issues, or we can have a call in zoom.