[PATCH dwarves v2 3/3] btf_encoder: Set .BTF section alignment to 16

From: Giuliano Procida <gprocida@google.com>
To: dwarves@vger.kernel.org
Cc: kernel-team@android.com, maennich@google.com, ast@kernel.org,
	andrii@kernel.org, bpf@vger.kernel.org, kernel-team@fb.com,
	Giuliano Procida <gprocida@google.com>
Subject: [PATCH dwarves v2 3/3] btf_encoder: Set .BTF section alignment to 16
Date: Thu, 21 Jan 2021 11:35:20 +0000	[thread overview]
Message-ID: <20210121113520.3603097-4-gprocida@google.com> (raw)
In-Reply-To: <20210121113520.3603097-1-gprocida@google.com>

NOTE: Do not apply. I will try to eliminate the dependency on objcopy
instead and achieve what's needed directly using libelf.

This is to avoid misaligned access when memory-mapping ELF sections.

Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 libbtf.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/libbtf.c b/libbtf.c
index 7552d8e..2f12d53 100644
--- a/libbtf.c
+++ b/libbtf.c
@@ -797,6 +797,14 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 			goto unlink;
 		}
 
+		snprintf(cmd, sizeof(cmd), "%s --set-section-alignment .BTF=16 %s",
+			 llvm_objcopy, filename);
+		if (system(cmd)) {
+			/* non-fatal, this is a nice-to-have and it's only supported from LLVM 10 */
+			fprintf(stderr, "%s: warning: failed to align .BTF section in '%s': %d!\n",
+				__func__, filename, errno);
+		}
+
 		err = 0;
 	unlink:
 		unlink(tmp_fn);
-- 
2.30.0.296.g2bfb1c46d8-goog

