[Bug preprocessor/58379] New: default mmap based implementation (mmap_gt_pch_get_address/mmap_gt_pch_use_address) is useless

[Bug preprocessor/58379] New: default mmap based implementation (mmap_gt_pch_get_address/mmap_gt_pch_use_address) is useless

[martin at netbsd dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58379

            Bug ID: 58379
           Summary: default mmap based implementation
                    (mmap_gt_pch_get_address/mmap_gt_pch_use_address) is
                    useless
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: martin at netbsd dot org

I may be misunderstanding the interface - but it looks to me like it lets the
kernel chose an arbitrary mapping address for different compiler invocations
but relies on the assumption that the returned address will be the same. If
not, the compiler fails with a fatal_error when trying to read a precompiled
header file ("had to relocate PCH").

I can not imagine a host system where this would work reliably (or even
typically twice in a row).

Please tell me I misunderstood, or consider disabling PCH support for host
platforms without host_hooks overriding this function.

[Bug preprocessor/58379] default mmap based implementation (mmap_gt_pch_get_address/mmap_gt_pch_use_address) is useless

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58379

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, it doesn't _rely_ on it - it basically makes systems where that is the
case work out of the box (every system pre address-space-randomization area).

If you have a system that randomizes then you have to re-define the hook.

[Bug preprocessor/58379] default mmap based implementation (mmap_gt_pch_get_address/mmap_gt_pch_use_address) is useless

[martin at netbsd dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58379

--- Comment #2 from Martin Husemann <martin at netbsd dot org> ---
(In reply to Richard Biener from comment #1)
> If you have a system that randomizes then you have to re-define the hook.

Besides ASLR there are various things out of control of the compiler that do
result in varying mapping adresses (like malloc using mmap instead of brk), so
chances are low in any modern system.

I'm not opposed to create a hook for NetBSD, but I have a hard time seeing a
possible sensible implementation. Look at the #ifdef cascade in
config/host-openbsd.c for a disgusting example of code that should not be in a
compiler (IMHO).

How hard is making the externalized format address neutral?

[Bug preprocessor/58379] default mmap based implementation (mmap_gt_pch_get_address/mmap_gt_pch_use_address) is useless

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58379

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=71934

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think this is getting fixed via PR 71934.