Re: [PATCH bpf-next v1 06/11] selftests/bpf: exercise the path ancestor iterator

["Jose E. Marchesi" <jemarch@gnu.org>]

> On Tue, Oct 13, 2026 at 12: 47: 55PM -0400, Justin Suess wrote: >
> Walk the ancestry of a mkdir's parent directory from a sleepable LSM
>> program and check that the iteration reaches the root: the
> temporary > directory, its parent and
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> On Tue, Oct 13, 2026 at 12:47:55PM -0400, Justin Suess wrote:
>> Walk the ancestry of a mkdir's parent directory from a sleepable LSM
>> program and check that the iteration reaches the root: the temporary
>> directory, its parent and / are visited, no position comes back
>> flagged, and the flags read back after the loop show it ended at the
>> root rather than on a failed allocation.
>> 
>> Hold the first position across the rest of the walk and resolve its
>> pathname only once the loop is done and the iterator destroyed: the
>> acquired reference, not the walk's, is then all that keeps the
>> position alive.
>> 
>> Signed-off-by: Justin Suess <utilityemal77@gmail.com>
>> ---
>>  .../selftests/bpf/prog_tests/path_ancestors.c | 57 ++++++++++++++
>>  .../selftests/bpf/progs/path_ancestors.c      | 74 +++++++++++++++++++
>>  2 files changed, 131 insertions(+)
>>  create mode 100644 tools/testing/selftests/bpf/prog_tests/path_ancestors.c
>>  create mode 100644 tools/testing/selftests/bpf/progs/path_ancestors.c
>> 
>> diff --git a/tools/testing/selftests/bpf/prog_tests/path_ancestors.c b/tools/testing/selftests/bpf/prog_tests/path_ancestors.c
>> new file mode 100644
>> index 000000000000..d1bd26bce10b
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/prog_tests/path_ancestors.c
>> @@ -0,0 +1,57 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/* Copyright (c) 2026 Justin Suess */
>> +
>> +#include <sys/stat.h>
>> +#include <stdlib.h>
>> +#include <unistd.h>
>> +#include <test_progs.h>
>> +#include "path_ancestors.skel.h"
>> +
>> +void test_path_ancestors(void)
>> +{
>> +     char base[] = "/tmp/path_ancestors_XXXXXX";
>> +     struct path_ancestors *skel = NULL;
>> +     char suba[280], subb[280];
>> +
>> +     if (!ASSERT_OK_PTR(mkdtemp(base), "mkdtemp"))
>> +             return;
>> +     snprintf(suba, sizeof(suba), "%s/a", base);
>> +     snprintf(subb, sizeof(subb), "%s/a/b", base);
>> +     if (!ASSERT_OK(mkdir(suba, 0755), "mkdir_a"))
>> +             goto out_rm;
>> +
>> +     skel = path_ancestors__open_and_load();
>> +     if (!ASSERT_OK_PTR(skel, "open_and_load"))
>> +             goto out_rm;
>> +     skel->bss->monitored_pid = getpid();
>> +     if (!ASSERT_OK(path_ancestors__attach(skel), "attach"))
>> +             goto out;
>> +
>> +     /* mkdir b: the hook sees dir == suba, whose ancestry is walked. */
>> +     if (!ASSERT_OK(mkdir(subb, 0755), "mkdir_b"))
>> +             goto out;
>> +
>> +     /* suba, base, /tmp, / at least. */
>> +     ASSERT_GE(skel->bss->ref_count, 4, "ref_count");
>> +     ASSERT_EQ(skel->bss->ref_flags, 0, "ref_flags");
>> +     /*
>> +      * The referenced loop ended at the root, not on an allocation
>> +      * failure.
>> +      */
>> +     ASSERT_EQ(skel->bss->end_flags, 0, "end_flags");
>> +
>> +     /* The acquired second position, used after its step was taken. */
>> +     ASSERT_STREQ(skel->bss->second_path, base, "second_path");
>> +     ASSERT_EQ(skel->bss->second_len, strlen(base) + 1, "second_len");
>> +
>> +     /* The first position, used only after the walk itself was gone. */
>> +     ASSERT_STREQ(skel->bss->first_path, suba, "first_path");
>> +     ASSERT_EQ(skel->bss->first_len, strlen(suba) + 1, "first_len");
>> +
>> +out:
>> +     path_ancestors__destroy(skel);
>> +out_rm:
>> +     rmdir(subb);
>> +     rmdir(suba);
>> +     rmdir(base);
>> +}
>> diff --git a/tools/testing/selftests/bpf/progs/path_ancestors.c b/tools/testing/selftests/bpf/progs/path_ancestors.c
>> new file mode 100644
>> index 000000000000..6287f5099a18
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/progs/path_ancestors.c
>> @@ -0,0 +1,74 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/* Copyright (c) 2026 Justin Suess */
>> +
>> +#include "vmlinux.h"
>> +#include <errno.h>
>> +#include <bpf/bpf_helpers.h>
>> +#include <bpf/bpf_tracing.h>
>> +#include "bpf_kfuncs.h"
>> +
>> +char _license[] SEC("license") = "GPL";
>> +
>> +__u32 monitored_pid;
>> +
>> +int ref_count;               /* positions seen by the pure referenced walk */
>> +int ref_flags;               /* pos flags seen by the referenced walk */
>> +int end_flags;               /* flags after a referenced loop ended: 0 unless NOMEM */
>> +int second_len;              /* d_path length of the walk's second position */
>> +int first_len;               /* d_path length of the first position, after the walk */
>> +char second_path[256];
>> +char first_path[256];
>> +
>> +static bool monitored(void)
>> +{
>> +     return (bpf_get_current_pid_tgid() >> 32) == monitored_pid;
>> +}
>> +
>> +SEC("lsm.s/path_mkdir")
>> +int BPF_PROG(walk_modes, const struct path *dir, struct dentry *dentry,
>> +          umode_t mode)
>> +{
>> +     struct bpf_iter_path_ancestors it;
>> +     struct path *pos, *first = NULL;
>> +
>> +     /*
>> +      * Keep first's init opaque: gcc-bpf otherwise folds it into a
>> +      * preceding loop's exit value, which the verifier cannot track.
>> +      */
>> +     barrier_var(first);
>> +
> cc'ing Eduard Zingerman + Jose Marchesi
>
> This barrier_var line was necessary for this selftest to pass
> under GCC BPF.
>
> Othwise it will be rejected by the verifier:
>
>   84: (85) call bpf_iter_path_ancestors_rcu_next#63514
>   85: R0=rcu_ptr_path(id=30) ... depth=9
>   86: (bf) r6 = r0              ; R6=rcu_ptr_path(id=30)
>   88: (55) if r0 != 0x0 goto pc-64
>   ...
>   101: (85) call bpf_rcu_read_unlock#64876
>        ^-- R6 downgraded rcu_ptr_ -> untrusted_ptr_
>   ...
>   146: (15) if r6 == 0x0 goto pc+10     ; R6=untrusted_ptr_path(id=30)
>   147: (bf) r1 = r6
>   151: (85) call bpf_path_d_path#64649
>   R1 must be referenced or trusted
>
> I think it's trying to optimize out the "struct path* first" and make it
> use the r6 register. It tries reusing the R6 register between the two
> loops, and doing so causes the program to fail to verify because it doesn't
> understand that the path was reinitialized with r6=0 from the previous
> exit.
>
> So I had to add the memory barrier to force struct path to be
> instantiated outside the register.
>
> There's nothing wrong from a *correctness* perspective with the code
> generated by GCC-BPF. But I'm not sure if this is a GCC-BPF bug or a
> bug in the SCEV widening series (merge 15b578b1715d). It depends on
> the contract BPF bytecode is supposed to meet.
>
> But clang works without the barrier, it's only GCC-BPF that produces
> this issue requiring the barrier.
>
> I can give more details if needed.

Please open a bugzilla at https://gcc.gnu.org/bugzilla so we can keep
track of this.  It would help to have a preprocessed reproducer based on
the selftest that could be used as a testcase in the GCC testsuite.

Thanks!

>
> Justin
>> [...]