[PATCH dwarves v2 4/4] btf_encoder: Align .BTF section/segment to 8 bytes

[Giuliano Procida <gprocida@google.com>]

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

Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 libbtf.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/libbtf.c b/libbtf.c
index 048a873..ae99a93 100644
--- a/libbtf.c
+++ b/libbtf.c
@@ -755,7 +755,13 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 	 * This actually happens in practice with vmlinux which has .strtab
 	 * after .shstrtab, resulting in a (small) hole the size of the original
 	 * .shstrtab.
+	 *
+	 * We'll align .BTF to 8 bytes to cater for all architectures. It'd be
+	 * nice if we could fetch this value from somewhere. The BTF
+	 * specification does not discuss alignment and its trailing string
+	 * table is not currently padded to any particular alignment.
 	 */
+	const size_t btf_alignment = 8;
 
 	/*
 	 * First we look if there was already a .BTF section present and
@@ -847,8 +853,8 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 	elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
 
 	/* Update .BTF section in the SHT */
-	size_t new_btf_offset = high_water_mark;
-	size_t new_btf_size = raw_btf_size;
+	size_t new_btf_offset = roundup(high_water_mark, btf_alignment);
+	size_t new_btf_size = roundup(raw_btf_size, btf_alignment);
 	GElf_Shdr btf_shdr_mem;
 	GElf_Shdr *btf_shdr = gelf_getshdr(btf_scn, &btf_shdr_mem);
 	if (!btf_shdr) {
@@ -856,6 +862,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 			__func__, elf_errmsg(elf_errno()));
 		goto out;
 	}
+	btf_shdr->sh_addralign = btf_alignment;
 	btf_shdr->sh_entsize = 0;
 	btf_shdr->sh_flags = SHF_ALLOC;
 	if (dot_btf_offset)
@@ -926,6 +933,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 		pht[phnum].p_memsz = pht[phnum].p_filesz = btf_shdr->sh_size;
 		pht[phnum].p_vaddr = pht[phnum].p_paddr = 0;
 		pht[phnum].p_flags = PF_R;
+		pht[phnum].p_align = btf_alignment;
 		void *phdr = gelf_newphdr(elf, phnum+1);
 		if (!phdr) {
 			fprintf(stderr, "%s: gelf_newphdr failed: %s\n",
-- 
2.30.0.365.g02bc693789-goog