bpf: introduce new bpf prog load flags "BPF_F_TEST_RND_HI32" [Linux 5.3]

This Linux kernel change "bpf: introduce new bpf prog load flags “BPF_F_TEST_RND_HI32″" is included in the Linux 5.3 release. This change is authored by Jiong Wang <jiong.wang [at] netronome.com> on Fri May 24 23:25:16 2019 +0100. The commit for this change in Linux stable tree is c240eff (patch).

bpf: introduce new bpf prog load flags "BPF_F_TEST_RND_HI32"

x86_64 and AArch64 perhaps are two arches that running bpf testsuite
frequently, however the zero extension insertion pass is not enabled for
them because of their hardware support.

It is critical to guarantee the pass correction as it is supposed to be
enabled at default for a couple of other arches, for example PowerPC,
SPARC, arm, NFP etc. Therefore, it would be very useful if there is a way
to test this pass on for example x86_64.

The test methodology employed by this set is "poisoning" useless bits. High
32-bit of a definition is randomized if it is identified as not used by any
later insn. Such randomization is only enabled under testing mode which is
gated by the new bpf prog load flags "BPF_F_TEST_RND_HI32".

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

There are 22 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 include/uapi/linux/bpf.h | 18 ++++++++++++++++++
 kernel/bpf/syscall.c     |  4 +++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 68d4470..7c6aef2 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -260,6 +260,24 @@ enum bpf_attach_type {
  */
 #define BPF_F_ANY_ALIGNMENT    (1U << 1)

+/* BPF_F_TEST_RND_HI32 is used in BPF_PROG_LOAD command for testing purpose.
+ * Verifier does sub-register def/use analysis and identifies instructions whose
+ * def only matters for low 32-bit, high 32-bit is never referenced later
+ * through implicit zero extension. Therefore verifier notifies JIT back-ends
+ * that it is safe to ignore clearing high 32-bit for these instructions. This
+ * saves some back-ends a lot of code-gen. However such optimization is not
+ * necessary on some arches, for example x86_64, arm64 etc, whose JIT back-ends
+ * hence hasn't used verifier's analysis result. But, we really want to have a
+ * way to be able to verify the correctness of the described optimization on
+ * x86_64 on which testsuites are frequently exercised.
+ *
+ * So, this flag is introduced. Once it is set, verifier will randomize high
+ * 32-bit for those instructions who has been identified as safe to ignore them.
+ * Then, if verifier is not doing correct analysis, such randomization will
+ * regress tests to expose bugs.
+ */
+#define BPF_F_TEST_RND_HI32    (1U << 2)
+
 /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
  * two extensions:
  *
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index cb5440b..3d546b6 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1604,7 +1604,9 @@ static int bpf_prog_load(union bpf_attr *attr, union bpf_attr __user *uattr)
    if (CHECK_ATTR(BPF_PROG_LOAD))
        return -EINVAL;

-   if (attr->prog_flags & ~(BPF_F_STRICT_ALIGNMENT | BPF_F_ANY_ALIGNMENT))
+   if (attr->prog_flags & ~(BPF_F_STRICT_ALIGNMENT |
+                BPF_F_ANY_ALIGNMENT |
+                BPF_F_TEST_RND_HI32))
        return -EINVAL;

    if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&

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