Dan Williams eab6870fee x86/spectre_v1: Disable compiler optimizations over array_index_mask_nospec()
Mark Rutland noticed that GCC optimization passes have the potential to elide
necessary invocations of the array_index_mask_nospec() instruction sequence,
so mark the asm() volatile.

Mark explains:

"The volatile will inhibit *some* cases where the compiler could lift the
 array_index_nospec() call out of a branch, e.g. where there are multiple
 invocations of array_index_nospec() with the same arguments:

        if (idx < foo) {
                idx1 = array_idx_nospec(idx, foo)
                do_something(idx1);
        }

        < some other code >

        if (idx < foo) {
                idx2 = array_idx_nospec(idx, foo);
                do_something_else(idx2);
        }

 ... since the compiler can determine that the two invocations yield the same
 result, and reuse the first result (likely the same register as idx was in
 originally) for the second branch, effectively re-writing the above as:

        if (idx < foo) {
                idx = array_idx_nospec(idx, foo);
                do_something(idx);
        }

        < some other code >

        if (idx < foo) {
                do_something_else(idx);
        }

 ... if we don't take the first branch, then speculatively take the second, we
 lose the nospec protection.

 There's more info on volatile asm in the GCC docs:

   https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
 "

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: babdde2698d4 ("x86: Implement array_index_mask_nospec")
Link: https://lkml.kernel.org/lkml/152838798950.14521.4893346294059739135.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2018-06-21 16:00:21 +02:00
2018-06-17 05:37:55 +09:00
2018-06-15 18:10:01 -03:00
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2018-06-13 08:40:34 -07:00
2018-06-15 07:55:25 +09:00
2018-06-15 18:10:01 -03:00
2018-06-15 07:55:25 +09:00
2018-06-12 13:11:26 -07:00
2018-06-15 18:10:01 -03:00
2018-06-15 18:10:01 -03:00
2018-04-15 17:21:30 -07:00
2017-11-17 17:45:29 -08:00
2018-06-17 08:04:49 +09:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
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Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
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    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.
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