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

Signed-off-by: Giuliano Procida <redacted>
---
 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