On 14 August 2017 at 16:32, Dave Martin [off-list ref] wrote:

On Mon, Aug 14, 2017 at 01:53:43PM +0100, Ard Biesheuvel wrote:

quoted

Like arm64, ARM supports position independent code sequences that
produce symbol references with a greater reach than the ordinary
adr/ldr instructions.

Currently, we use open coded instruction sequences involving literals
and arithmetic operations. Instead, we can use movw/movt pairs on v7
CPUs, circumventing the D-cache entirely. For older CPUs, we can emit
the literal into a subsection, allowing it to be emitted out of line
while retaining the ability to perform arithmetic on label offsets.

E.g., on pre-v7 CPUs, we can emit a PC-relative reference as follows:

       ldr          <reg>, 222f
  111: add          <reg>, <reg>, pc
       .subsection  1
  222: .long        <sym> - (111b + 8)
       .previous

This is allowed by the assembler because, unlike ordinary sections,
subsections are combined into a single section into the object file,
and so the label references are not true cross-section references that
are visible as relocations. Note that we could even do something like

       add          <reg>, pc, #(222f - 111f) & ~0xfff
       ldr          <reg>, [<reg>, #(222f - 111f) & 0xfff]
  111: add          <reg>, <reg>, pc
       .subsection  1
  222: .long        <sym> - (111b + 8)
       .previous

if it turns out that the 4 KB range of the ldr instruction is insufficient
to reach the literal in the subsection, although this is currently not a
problem (of the 98 objects built from .S files in a multi_v7_defconfig
build, only 11 have .text sections that are over 1 KB, and the largest one
[entry-armv.o] is 3308 bytes)

Subsections have been available in binutils since 2004 at least, so
they should not cause any issues with older toolchains.

So use the above to implement the macros mov_l, adr_l, adrm_l (using ldm

I don't see adrm_l in this patch.

Oops (2)

Nico already mentioned that, and I failed to fix the commit log. I
added it at some point, but it wasn't really useful

quoted

to load multiple literals at once), ldr_l and str_l, all of which will
use movw/movt pairs on v7 and later CPUs, and use PC-relative literals
otherwise.

Also...

By default, I'd assume that we should port _all_ uses of :upper16:/
:lower16: to use these.  Does this series consciously do that?  Are
there any exceptions?

There aren't that many. Anything that refers to absolute symbols will
break under CONFIG_RELOCATABLE and I haven't noticed any issues (I
tested extensively with Thumb2)

I don't mind open coded movw/movt for relative references in code that
is tightly coupled to a platform that guarantees v7+ so I didn't do a
full sweep. Also, I started with 50+ patches and tried to remove the
ones that are mostly orthogonal to the KASLR stuff.