elf obj load: skip zero-sized sections early

[Bjoern A. Zeeb]

On Fri, 25 Jun 2010, Andriy Gapon wrote:

Hey,

> Proposed patch skips zero sized sections without going into trouble of
> allocating section entry (progtab), doing zero-sized memory allocs and copies.
> I observe that sometimes zero-sized set_pcpu sections are produced in module
> objects, maybe when a module doesn't create any per cpu data of its one, but
> references external pcpu data.  Not sure.
>
> Main goal of this patch is to play nice with external debugging tools (e.g.
> kgdb) which ignore zero-sized sections outright.  Current code effectively
> ignores but may apply their alignment to the next non-zero section if its
> required alignment is smaller.  So the patch should get rid of that side effect
> as well as do some very tiny resource conservation.
>
> This work is based on np@'s investigation and original patch:
> http://lists.freebsd.org/pipermail/freebsd-hackers/2009-November/030093.html

Have you guys figured this out already?

Adding kib to Cc: in case he can help.

/bz


> 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:
> diff --git a/sys/kern/link_elf_obj.c b/sys/kern/link_elf_obj.c
> index a337fd0..b0df57d 100644
> --- a/sys/kern/link_elf_obj.c
> +++ b/sys/kern/link_elf_obj.c
> @@ -555,6 +555,8 @@ link_elf_load_file(linker_class_t cls, const char *filename,
> 	symtabindex = -1;
> 	symstrindex = -1;
> 	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:
> @@ -677,6 +679,8 @@ link_elf_load_file(linker_class_t cls, const char *filename,
> 	/* Size up code/data(progbits) and bss(nobits). */
> 	alignmask = 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:
> @@ -737,6 +741,8 @@ link_elf_load_file(linker_class_t cls, const char *filename,
> 	ra = 0;
> 	alignmask = 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:
>
>

-- 
Bjoern A. Zeeb    From August on I will have a life.  It's now up to you
to do the maths and count to 64.     -- Bondorf, Germany, 14th June 2010