Re: [PATCH] tracing: properly align linker defined symbols

[Linus Torvalds]

On Fri, Jul 9, 2010 at 11:35 PM, Sam Ravnborg <sam@xxxxxxxxxxxx> wrote:
>
> We define a number of symbols in the linker scipt like this:
>
>    __start_syscalls_metadata = .;
>    *(__syscalls_metadata)
>
> But we do not know the alignment of "." when we assign
> the __start_syscalls_metadata symbol.
> gcc started to uses bigger alignment for structs (32 bytes),
> so we saw situations where the linker due to alignment
> constraints increased the value of "." after the symbol assignment.

Ok, why not clean this up a bit more, and use a helper macro for this
pattern. There's a fair number of users of that kind of pattern, so
that actually removes a few lines.

Here's an example patch. Untested. Whatever. But just this part

 1 files changed, 21 insertions(+), 31 deletions(-)

says to me that it's a good idea, and there are other cases that could
use the new SYMBOL_SECTION() helper.

What do people think?

                       Linus
>
> This resulted in boot fails.
>
> Fix this by forcing a 32 byte alignment of "." before the
> assignment.
>
> This patch introduces the forced alignment for
> ftrace_events and syscalls_metadata.
> It may be required in more places.
>
> Reported-by: Zeev Tarantov <zeev.tarantov@xxxxxxxxx>
> Signed-off-by: Sam Ravnborg <sam@xxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> ---
>  include/asm-generic/vmlinux.lds.h |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 48c5299..4b5902a 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -63,6 +63,12 @@
>  /* Align . to a 8 byte boundary equals to maximum function alignment. */
>  #define ALIGN_FUNCTION()  . = ALIGN(8)
>
> +/*
> + * Align to a 32 byte boundary equal to the
> + * alignment gcc 4.5 uses for a struct
> + */
> +#define STRUCT_ALIGN() . = ALIGN(32)
> +
>  /* The actual configuration determine if the init/exit sections
>  * are handled as text/data or they can be discarded (which
>  * often happens at runtime)
> @@ -166,7 +172,11 @@
>        LIKELY_PROFILE()                                                \
>        BRANCH_PROFILE()                                                \
>        TRACE_PRINTKS()                                                 \
> +                                                                       \
> +       STRUCT_ALIGN();                                                 \
>        FTRACE_EVENTS()                                                 \
> +                                                                       \
> +       STRUCT_ALIGN();                                                 \
>        TRACE_SYSCALLS()
>
>  /*
> --
> 1.6.0.6
>
>

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