[PATCH] jump_label: align jump_entry table to at least 4-bytes

[David Daney]

On 02/27/2017 01:06 PM, Steven Rostedt wrote:
> On Mon, 27 Feb 2017 11:59:50 -0800
> David Daney <ddaney at caviumnetworks.com> wrote:
>
>> For me the size is not the important issue, it is the alignment of the
>> struct jump_entry entries in the table.  I don't understand how your
>> patch helps, and I cannot Acked-by unless I understand what is being
>> done and can see that it is both correct and necessary.
>
> You brought up a very good point and I'm glad that I had Jason Cc all
> the arch maintainers in one patch.
>
> I think jump_labels may be much more broken than we think, and Jason's
> fix doesn't fix anything. We had this same issues with tracepoints.
>
> I'm looking at jump_label_init, and how we iterate over an array of
> struct jump_entry's that was put together by the linker. The problem is
> that jump_entry is not a power of 2 in size.
>

ELF sections may have an ENTSIZE property exactly for arrays.  Since 
each jump_entry will have a unique value they cannot be merged, but we 
can tell the assembler they are an array and get them properly packed. 
Perhaps something like (untested):

    .pushsection __jump_table,  \"awM\", at progbits,24
    FOO
    .popsection


> struct jump_entry {
> 	jump_label_t code;
> 	jump_label_t target;
> 	jump_label_t key;
> };
>
> When putting together arrays of this kind, the linker is in its right
> to add padding for alignment, in the middle of the array! It has no
> idea that this is an array, and there's nothing stopping the linker
> from messing it up.
>
> For those structs that are a power of 2 in size, there's no reason for
> the linker to do anything else, and it "just works". There's plenty of
> instances in the kernel that depend on this.
>
> I'm thinking that the sort algorithm either hid the problem or fixed it
> somehow (I'm guessing it hid the problem).
>
> I hit the same issue with trace event structures. The solution was to
> create the array of pointers to each structure, and dereference the
> structures from the array.
>
> See commit e4a9ea5ee ("tracing: Replace trace_event struct array with
> pointer array")
>
> -- Steve
>