[PATCH bpf-next v2] selftests/bpf: Fix compilation errors: assign a value to a constant

[Rong Tao <rtoax@xxxxxxxxxxx>]

From: Rong Tao <rongtao@xxxxxxxx>

commit bc292ab00f6c("mm: introduce vma->vm_flags wrapper functions")
turns the vm_flags into a const variable.

Added bpf_find_vma test in commit f108662b27c9("selftests/bpf: Add tests
for bpf_find_vma") to assign values to variables that declare const in
find_vma_fail1.c programs, which is an error to the compiler and does not
test BPF verifiers. It is better to replace 'const vm_flags_t vm_flags'
with 'unsigned long vm_start' for testing.

    $ make -C tools/testing/selftests/bpf/ -j8
    ...
    progs/find_vma_fail1.c:16:16: error: cannot assign to non-static data
    member 'vm_flags' with const-qualified type 'const vm_flags_t' (aka
    'const unsigned long')
            vma->vm_flags |= 0x55;
            ~~~~~~~~~~~~~ ^
    ../tools/testing/selftests/bpf/tools/include/vmlinux.h:1898:20:
    note: non-static data member 'vm_flags' declared const here
                    const vm_flags_t vm_flags;
                    ~~~~~~~~~~~`~~~~~~^~~~~~~~

Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
---
v2: Add more useful commit information
v1: https://lore.kernel.org/lkml/tencent_FC8827062142CF5936974B2A30AF6CA3C408@xxxxxx/
---
 tools/testing/selftests/bpf/progs/find_vma_fail1.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/find_vma_fail1.c b/tools/testing/selftests/bpf/progs/find_vma_fail1.c
index b3b326b8e2d1..47d5dedff554 100644
--- a/tools/testing/selftests/bpf/progs/find_vma_fail1.c
+++ b/tools/testing/selftests/bpf/progs/find_vma_fail1.c
@@ -13,7 +13,7 @@ static long write_vma(struct task_struct *task, struct vm_area_struct *vma,
 		      struct callback_ctx *data)
 {
 	/* writing to vma, which is illegal */
-	vma->vm_flags |= 0x55;
+	vma->vm_start = 0xffffffffff600000;
 
 	return 0;
 }
-- 
2.39.2