On Mon, Oct 2, 2017 at 1:08 PM, Segher Boessenkool
[off-list ref] wrote:

On Mon, Oct 02, 2017 at 12:29:45PM -0700, Kees Cook wrote:

quoted

On Mon, Sep 25, 2017 at 12:41 PM, Segher Boessenkool
[off-list ref] wrote:

quoted

On Mon, Sep 25, 2017 at 04:01:55PM +0000, David Laight wrote:

quoted

From: Segher Boessenkool

quoted

The compiler puts this item in .sdata, for 32-bit.  There is no .srodata,
so if it wants to use a small data section, it must use .sdata .

Non-external, non-referenced symbols are not put in .sdata, that is the
difference you see with the "static".

I don't think there is a bug here.  If you think there is, please open
a GCC bug.

The .sxxx sections are for 'small' data that can be accessed (typically)
using small offsets from a global register.
This means that all sections must be adjacent in the image.
So you can't really have readonly small data.

My guess is that the linker script is putting .srodata in with .sdata.

.srodata does not *exist* (in the ABI).

So, I still think this is a bug. The variable is marked const: this is
not a _suggestion_. :) If the compiler produces output where the
variable is writable, that's a bug.

C11 6.7.3/6: "If an attempt is made to modify an object defined with a
const-qualified type through use of an lvalue with non-const-qualified
type, the behavior is undefined."

And that is all that "const" means.

The compiler is free to put this var in *no* data section, or to copy
it to the stack before using it, or anything else it thinks is a good
idea.

The kernel depends on const things being read-only. I realize C11 says
this is "undefined", but from a kernel security perspective, const
means read-only, and this is true on other architectures. Now,
strictly speaking, the compiler is just responsible for doing section
assignment for a variable, and the linker then lays things out, but
the result carries the requested memory protections (i.e. read-only,
executable, etc). If "const" is just a hint, then what is the
canonical way to have gcc put a variable into a section that the
linker will always request be kept read-only?

If you think it would be a good idea for the compiler to change its
behaviour here, please file a PR (or send a patch).  Please bring
arguments why we would want to change this.

Sure:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82411

quoted

I can't tell if this bug is related:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=9571

I don't think so: the only remaining bug there is that a copy of the
constant is put in .rodata.cst8 (although there is a copy in .sdata2
already).

Okay, thanks for checking.

-Kees

-- 
Kees Cook
Pixel Security