Re: [RFC] kexec_file: Add support for purgatory built as PIE

Hello Baoquan,

Am Freitag, 4. November 2016, 15:38:40 BRST schrieb Baoquan He:

On 11/02/16 at 04:00am, Thiago Jung Bauermann wrote:

quoted

Hello,

The kexec_file code currently builds the purgatory as a partially linked
object (using ld -r). Is there a particular reason to use that instead of
a position independent executable (PIE)?

It's taken as "-r", relocatable in user space kexec-tools too originally.
I think Vivek just keeps it the same when moving into kernel.

Ok. If that's the only reason then PIE is better suited at least for powerpc.

quoted

I found a discussion from 2013 in the archives but from what I understood
it was about the purgatory as a separate object vs having it linked into
the kernel, which is different from what I'm asking:

http://lists.infradead.org/pipermail/kexec/2013-December/010535.html

Here is my motivation for this question:
 On ppc64 purgatory.ro has 12 relocation types when built as a partially

linked object. This makes arch_kexec_apply_relocations_add duplicate a lot
of code with module_64.c:apply_relocate_add to implement these
relocations. The alternative is to do some refactoring so that both
functions can share the implementation of the relocations. This is done
in patches 5 and 6 of the
kexec_file_load implementation for powerpc:

In user space kexec-tools utility, you also got this problem?

Yes, kexec-tools' purgatory.ro has 10 relocation types instead of 12 (I don't 
know why), but that's still a lot.

quoted

@@ -942,7 +1085,13 @@ static Elf_Sym *kexec_purgatory_find_symbol(struct

purgatory_info *pi,> 
 		/* Go through symbols for a match */
 		for (k = 0; k < sechdrs[i].sh_size/sizeof(Elf_Sym); k++) {

-			if (ELF_ST_BIND(syms[k].st_info) != STB_GLOBAL)
+			/*
+			 * FIXME: See if we can or should export the .TOC.
+			 * symbol as global instead of searching local symbols
+			 * here.
+			 */
+			if (ELF_ST_BIND(syms[k].st_info) != STB_GLOBAL &&
+			    ELF_ST_BIND(syms[k].st_info) != STB_LOCAL)

 				continue;
 			
 			if (strcmp(strtab + syms[k].st_name, name) != 0)

I don't need the change above anymore. I found a way to obtain the TOC pointer 
without looking for the .TOC. symbol.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center