tools/bpftool: Generate data section struct with conservative alignment

The comment in the code describes this in good details. Generate such a memory
layout that would work both on 32-bit and 64-bit architectures for user-space.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200813204945.1020225-9-andriin@fb.com

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index db80e83..f611846 100644 (file)
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -143,6 +143,20 @@ static int codegen_datasec_def(struct bpf_object *obj,
                              var_name, align);
                        return -EINVAL;
                }
+               /* Assume 32-bit architectures when generating data section
+                * struct memory layout. Given bpftool can't know which target
+                * host architecture it's emitting skeleton for, we need to be
+                * conservative and assume 32-bit one to ensure enough padding
+                * bytes are generated for pointer and long types. This will
+                * still work correctly for 64-bit architectures, because in
+                * the worst case we'll generate unnecessary padding field,
+                * which on 64-bit architectures is not strictly necessary and
+                * would be handled by natural 8-byte alignment. But it still
+                * will be a correct memory layout, based on recorded offsets
+                * in BTF.
+                */
+               if (align > 4)
+                       align = 4;
 
                align_off = (off + align - 1) / align * align;
                if (align_off != need_off) {