[PATCH v3 08/21] arm64: add support for module PLTs

On Fri, Jan 22, 2016 at 06:06:52PM +0100, Ard Biesheuvel wrote:

On 22 January 2016 at 17:55, Mark Rutland [off-list ref] wrote:

quoted

quoted

+static bool in_init(const struct module *mod, void *addr)
+{
+     return (u64)addr - (u64)mod->module_init < mod->init_size;
+}
+
+u64 get_module_plt(struct module *mod, void *loc, u64 val)
+{
+     struct plt_entry entry = {
+             cpu_to_le32(0x92800010 | (((~val      ) & 0xffff)) << 5),
+             cpu_to_le32(0xf2a00010 | ((( val >> 16) & 0xffff)) << 5),
+             cpu_to_le32(0xf2c00010 | ((( val >> 32) & 0xffff)) << 5),
+             cpu_to_le32(0xd61f0200)
+     }, *plt;

It would be nice if we could un-magic this, though I see that reusing
the existing insn or reloc_insn code is painful here.

Well, I could #define PLT0 PLT1 PLT2 etc, and document them a bit
better, but having all the instruction machinery for emitting the
exact same instructions each time seems a bit overkill imo.

Well, almost the same (the target address does change after all).

I agree that this looks more complicated using the insn machinery, based
on local experimentation. Oh well...
 

quoted

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+     int i, *count;
+
+     if (in_init(mod, loc)) {
+             plt = (struct plt_entry *)mod->arch.init_plt->sh_addr;
+             count = &mod->arch.init_plt_count;
+     } else {
+             plt = (struct plt_entry *)mod->arch.core_plt->sh_addr;
+             count = &mod->arch.core_plt_count;
+     }
+
+     /* Look for an existing entry pointing to 'val' */
+     for (i = 0; i < *count; i++)
+             if (plt[i].mov0 == entry.mov0 &&
+                 plt[i].mov1 == entry.mov1 &&
+                 plt[i].mov2 == entry.mov2)
+                     return (u64)&plt[i];

I think that at the cost of redundantly comparing the br x16, you could
simplify this by comparing the whole struct, e.g.

        for (i = 0; i < *count; i++)
                if (plt[i] == entry)

You can use struct types in assignments, but not in comparisons,
strangely enough

Ah, sorry for the noise.

quoted

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+     for (s = sechdrs + 1; s < sechdrs_end; ++s) {

Could we have a comment as to why we skip the first Shdr? I recall it's
in some way special, but I can't recall why/how.

I don't remember exactly, and some of this code originated on ia64 IIRC.
Probably better to simply start from [0]

Ok.

quoted

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+             const Elf64_Rela *rels = (void *)ehdr + s->sh_offset;
+             int numrels = s->sh_size / sizeof(Elf64_Rela);
+             Elf64_Shdr *dstsec = sechdrs + s->sh_info;
+
+             if (s->sh_type != SHT_RELA)
+                     continue;

We only have RELA, and no REL?

Nope.

arch/arm64/Kconfig:86:  select MODULES_USE_ELF_RELA

Evidently I didn't do enough background reading.

As I said, this code will look different in the next version, but I
will make sure to take your review points.

Cheers! :)

Mark.