[PATCH bpf] bpf: fix panic in stack_map_get_build_id() on i386 and arm32

[Song Liu]

As Naresh reported, test_stacktrace_build_id() causes panic on i386 and
arm32 systems. This is caused by page_address() returns NULL in certain
cases.

This patch fixes this error by using kmap_atomic/kunmap_atomic instead
of page_address.

Fixes: 615755a77b24 (" bpf: extend stackmap to save binary_build_id+offset 
instead of address")
Reported-by: Naresh Kamboju <naresh.kamb...@linaro.org>
Signed-off-by: Song Liu <songliubrav...@fb.com>
---
 kernel/bpf/stackmap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 90daf285de03..d9e2483669d0 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -260,7 +260,7 @@ static int stack_map_get_build_id(struct vm_area_struct 
*vma,
                return -EFAULT; /* page not mapped */
 
        ret = -EINVAL;
-       page_addr = page_address(page);
+       page_addr = kmap_atomic(page);
        ehdr = (Elf32_Ehdr *)page_addr;
 
        /* compare magic x7f "ELF" */
@@ -276,6 +276,7 @@ static int stack_map_get_build_id(struct vm_area_struct 
*vma,
        else if (ehdr->e_ident[EI_CLASS] == ELFCLASS64)
                ret = stack_map_get_build_id_64(page_addr, build_id);
 out:
+       kunmap_atomic(page_addr);
        put_page(page);
        return ret;
 }
-- 
2.17.1