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    SubjectRe: [PATCH][GIT PULL][for 2.6.35] tracing: Add alignment to syscall metadata declarations
    On Fri, Jul 09, 2010 at 05:05:50PM -0400, Steven Rostedt wrote:
    > On Fri, 2010-07-09 at 22:53 +0200, Sam Ravnborg wrote:
    >
    > > But in this case we have no control of the value of "." (current address)
    > > when we have processed (_ftrace_events) so it may even be at a 2 byte boundary.
    > > The linker will add padding as needed to satisfy the alignmnet of
    > > __syscalls_metadata - but that padding will be inbetween "." and the first
    > > member in __syscalls_metadata.
    >
    > Fine, but this is a separate issue.
    No - this is the exact issue we are facing here.

    In the linker script we have no control what-so-ever on the alignment of "."
    so relying on any alignment is a potential bug.

    In this particular case gcc decide to increase the alignment of
    syscall_metadata from 8 to 16.

    And then suddenly things fall apart because we rely on "." being
    properly aligned - and it is only 4 byte aligned.

    So you fixed this by forcing gcc to use a smaller alignment
    for syscall_metadata (from 16 => 4).
    But what happens next time when "." is two-byte aligned?
    We will not change aligment down to 2...

    So to fix this properly we need to make sure that "." is properly
    aligned to the alignment of syscall_metadata.

    In other words - the linker script needs to be fixed too.
    But the fix in the .h file is required so we know the alignmnet
    of syscall_metadata.

    Sam


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