completely ignore zero-sized elf sections in modules of elf object type (amd64)

Current code doesn't check size of elf sections and may perform needless
actions of zero-sized memory allocation and similar.
The bigger issue is that alignment requirement of a zero-sized section
gets effectively applied to the next section if it has smaller alignment
requirement.  But other tools, like gdb and consequently kgdb,
completely ignore zero-sized sections and thus may map symbols to
addresses differently.

Zero-sized sections are not typical in general.
Their typical (only, even) cause in FreeBSD modules is inline assembly that
creates custom sections which is found in pcpu.h and vnet.h.  Mere inclusion
of one of those header files produces a custom section in elf output.
If there is no actual use for the section in a given module, then the
section remains empty.

Better solution is to avoid creating zero-sized sections altogether,
which is in plans.

Preloaded modules are handled in boot code (load_elf_obj.c), while
dynamically loaded modules are handled by kernel (link_elf_obj.c).

Based on code by:	np
MFC after:		3 weeks

1 files changed, 2 insertions, 0 deletions

diff --git a/sys/boot/common/load_elf_obj.c b/sys/boot/common/load_elf_obj.c
index 4b3aaea..6f3b349 100644
--- a/sys/boot/common/load_elf_obj.c
+++ b/sys/boot/common/load_elf_obj.c
@@ -221,6 +221,8 @@ __elfN(obj_loadimage)(struct preloaded_file *fp, elf_file_t ef, u_int64_t off)
 	for (i = 0; i < hdr->e_shnum; i++)
 		shdr[i].sh_addr = 0;
 	for (i = 0; i < hdr->e_shnum; i++) {
+		if (shdr[i].sh_size == 0)
+			continue;
 		switch (shdr[i].sh_type) {
 		case SHT_PROGBITS:
 		case SHT_NOBITS: