[PATCH bpf v6 3/3] selftests/bpf: Test PROBE_MEM of VSYSCALL_ADDR on x86-64

[Puranjay Mohan <puranjay@kernel.org>]

The vsyscall is a legacy API for fast execution of system calls. It maps
a page at address VSYSCALL_ADDR into the userspace program. This address
is in the top 10MB of the address space:

ffffffffff600000 - ffffffffff600fff |    4 kB | legacy vsyscall ABI

The last commit fixes the x86-64 BPF JIT to skip accessing addresses in
this memory region. Add this address to bpf_testmod_return_ptr() so we
can make sure that it is fixed.

After this change and without the previous commit, subprogs_extable
selftest will crash the kernel.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
index 39ad96a18123..edcd26106557 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
@@ -205,6 +205,9 @@ __weak noinline struct file *bpf_testmod_return_ptr(int arg)
 	case 5: return (void *)~(1ull << 30);	/* trigger extable */
 	case 6: return &f;			/* valid addr */
 	case 7: return (void *)((long)&f | 1);	/* kernel tricks */
+#ifdef CONFIG_X86_64
+	case 8: return (void *)VSYSCALL_ADDR;   /* vsyscall page address */
+#endif
 	default: return NULL;
 	}
 }
-- 
2.40.1