[PATCH bpf-next] selftests/bpf: Fix alignment of .BTF_ids

[PATCH bpf-next] selftests/bpf: Fix alignment of .BTF_ids

[Jean-Philippe Brucker]

Fix a build failure on arm64, due to missing alignment information for
the .BTF_ids section:

resolve_btfids.test.o: in function `test_resolve_btfids':
tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140:(.text+0x29c): relocation truncated to fit: R_AARCH64_LDST32_ABS_LO12_NC against `.BTF_ids'
ld: tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140: warning: one possible cause of this error is that the symbol is being referenced in the indicated code as if it had a larger alignment than was declared where it was defined

In vmlinux, the .BTF_ids section is aligned to 4 bytes by vmlinux.lds.h.
In test_progs however, .BTF_ids doesn't have alignment constraints. The
arm64 linker expects the btf_id_set.cnt symbol, a u32, to be naturally
aligned but finds it misaligned and cannot apply the relocation. Enforce
alignment of .BTF_ids to 4 bytes.

Fixes: cd04b04de119 ("selftests/bpf: Add set test to resolve_btfids")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
---
 tools/testing/selftests/bpf/prog_tests/resolve_btfids.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
index 8826c652adad..6ace5e9efec1 100644
--- a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
+++ b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
@@ -28,6 +28,12 @@ struct symbol test_symbols[] = {
 	{ "func",    BTF_KIND_FUNC,    -1 },
 };
 
+/* Align the .BTF_ids section to 4 bytes */
+asm (
+".pushsection " BTF_IDS_SECTION " ,\"a\"; \n"
+".balign 4, 0;                            \n"
+".popsection;                             \n");
+
 BTF_ID_LIST(test_list_local)
 BTF_ID_UNUSED
 BTF_ID(typedef, S)
-- 
2.28.0

Re: [PATCH bpf-next] selftests/bpf: Fix alignment of .BTF_ids

[Jiri Olsa]

On Wed, Sep 30, 2020 at 11:36:01AM +0200, Jean-Philippe Brucker wrote:
> Fix a build failure on arm64, due to missing alignment information for
> the .BTF_ids section:
> 
> resolve_btfids.test.o: in function `test_resolve_btfids':
> tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140:(.text+0x29c): relocation truncated to fit: R_AARCH64_LDST32_ABS_LO12_NC against `.BTF_ids'
> ld: tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140: warning: one possible cause of this error is that the symbol is being referenced in the indicated code as if it had a larger alignment than was declared where it was defined
> 
> In vmlinux, the .BTF_ids section is aligned to 4 bytes by vmlinux.lds.h.
> In test_progs however, .BTF_ids doesn't have alignment constraints. The
> arm64 linker expects the btf_id_set.cnt symbol, a u32, to be naturally
> aligned but finds it misaligned and cannot apply the relocation. Enforce
> alignment of .BTF_ids to 4 bytes.
> 
> Fixes: cd04b04de119 ("selftests/bpf: Add set test to resolve_btfids")
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> ---
>  tools/testing/selftests/bpf/prog_tests/resolve_btfids.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
> index 8826c652adad..6ace5e9efec1 100644
> --- a/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
> +++ b/tools/testing/selftests/bpf/prog_tests/resolve_btfids.c
> @@ -28,6 +28,12 @@ struct symbol test_symbols[] = {
>  	{ "func",    BTF_KIND_FUNC,    -1 },
>  };
>  
> +/* Align the .BTF_ids section to 4 bytes */
> +asm (
> +".pushsection " BTF_IDS_SECTION " ,\"a\"; \n"
> +".balign 4, 0;                            \n"
> +".popsection;                             \n");
> +

Acked-by: Jiri Olsa <jolsa@redhat.com>

thanks,
jirka

>  BTF_ID_LIST(test_list_local)
>  BTF_ID_UNUSED
>  BTF_ID(typedef, S)
> -- 
> 2.28.0
> 

Re: [PATCH bpf-next] selftests/bpf: Fix alignment of .BTF_ids

[patchwork-bot+bpf]

Hello:

This patch was applied to bpf/bpf-next.git (refs/heads/master):

On Wed, 30 Sep 2020 11:36:01 +0200 you wrote:
> Fix a build failure on arm64, due to missing alignment information for
> the .BTF_ids section:
> 
> resolve_btfids.test.o: in function `test_resolve_btfids':
> tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140:(.text+0x29c): relocation truncated to fit: R_AARCH64_LDST32_ABS_LO12_NC against `.BTF_ids'
> ld: tools/testing/selftests/bpf/prog_tests/resolve_btfids.c:140: warning: one possible cause of this error is that the symbol is being referenced in the indicated code as if it had a larger alignment than was declared where it was defined
> 
> [...]

Here is the summary with links:
  - [bpf-next] selftests/bpf: Fix alignment of .BTF_ids
    https://git.kernel.org/bpf/bpf-next/c/3effc06a4dde

You are awesome, thank you!
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