Re: [PATCH v3] earlycon: Use a pointer table to fix __earlycon_table stride

[Guenter Roeck]

On Tue, Mar 20, 2018 at 3:02 PM, Daniel Kurtz <djkurtz@xxxxxxxxxxxx> wrote:
> Commit 99492c39f39f ("earlycon: Fix __earlycon_table stride") tried to fix
> __earlycon_table stride by forcing the earlycon_id struct alignment to 32
> and asking the linker to 32-byte align the __earlycon_table symbol.  This
> fix was based on commit 07fca0e57fca92 ("tracing: Properly align linker
> defined symbols") which tried a similar fix for the tracing subsystem.
>
> However, this fix doesn't quite work because there is no guarantee that
> gcc will place structures packed into an array format.  In fact, gcc 4.9
> chooses to 64-byte align these structs by inserting additional padding
> between the entries because it has no clue that they are supposed to be in
> an array.  If we are unlucky, the linker will assign symbol
> "__earlycon_table" to a 32-byte aligned address which does not correpsond
> to the 64-byte alignbed contents of section "__earlycon_table".
>
> To address this same problem, the fix to the tracing system was
> subsequently re-implemented using a more robust table of pointers approach
> by commits:
>  3d56e331b653 ("tracing: Replace syscall_meta_data struct array with pointer array")
>  654986462939 ("tracepoints: Fix section alignment using pointer array")
>  e4a9ea5ee7c8 ("tracing: Replace trace_event struct array with pointer array")
>
> Let's use this same "array of pointers to structs" approach for
> EARLYCON_TABLE.
>
> Fixes: 99492c39f39f ("earlycon: Fix __earlycon_table stride")
> Signed-off-by: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
> Suggested-by: Aaron Durbin <adurbin@xxxxxxxxxxxx>

Build tested on sh4, alpha, and h8300 to ensure that the build problem
reported by 0day is gone.
For that,

Tested-by: Guenter Roeck <groeck@xxxxxxxxxxxx>

I am not an expert in this area, but the change makes sense to me. As such,

Reviewed-by: Guenter Roeck <groeck@xxxxxxxxxxxx>

but take it with a grain of salt.

Guenter

> ---
> Changes since v2:
>  * Use __initconst instead of __initdata to avoid h8300 and alpha kbuild errors
>
>  drivers/of/fdt.c                  |  7 +++++--
>  drivers/tty/serial/earlycon.c     |  6 ++++--
>  include/asm-generic/vmlinux.lds.h |  2 +-
>  include/linux/serial_core.h       | 21 ++++++++++++++-------
>  4 files changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index 84aa9d676375..6da20b9688f7 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -942,7 +942,7 @@ int __init early_init_dt_scan_chosen_stdout(void)
>         int offset;
>         const char *p, *q, *options = NULL;
>         int l;
> -       const struct earlycon_id *match;
> +       const struct earlycon_id **p_match;
>         const void *fdt = initial_boot_params;
>
>         offset = fdt_path_offset(fdt, "/chosen");
> @@ -969,7 +969,10 @@ int __init early_init_dt_scan_chosen_stdout(void)
>                 return 0;
>         }
>
> -       for (match = __earlycon_table; match < __earlycon_table_end; match++) {
> +       for (p_match = __earlycon_table; p_match < __earlycon_table_end;
> +            p_match++) {
> +               const struct earlycon_id *match = *p_match;
> +
>                 if (!match->compatible[0])
>                         continue;
>
> diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> index a24278380fec..22683393a0f2 100644
> --- a/drivers/tty/serial/earlycon.c
> +++ b/drivers/tty/serial/earlycon.c
> @@ -169,7 +169,7 @@ static int __init register_earlycon(char *buf, const struct earlycon_id *match)
>   */
>  int __init setup_earlycon(char *buf)
>  {
> -       const struct earlycon_id *match;
> +       const struct earlycon_id **p_match;
>
>         if (!buf || !buf[0])
>                 return -EINVAL;
> @@ -177,7 +177,9 @@ int __init setup_earlycon(char *buf)
>         if (early_con.flags & CON_ENABLED)
>                 return -EALREADY;
>
> -       for (match = __earlycon_table; match < __earlycon_table_end; match++) {
> +       for (p_match = __earlycon_table; p_match < __earlycon_table_end;
> +            p_match++) {
> +               const struct earlycon_id *match = *p_match;
>                 size_t len = strlen(match->name);
>
>                 if (strncmp(buf, match->name, len))
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 1ab0e520d6fc..e17de55c2542 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -179,7 +179,7 @@
>  #endif
>
>  #ifdef CONFIG_SERIAL_EARLYCON
> -#define EARLYCON_TABLE() STRUCT_ALIGN();                       \
> +#define EARLYCON_TABLE() . = ALIGN(8);                         \
>                          VMLINUX_SYMBOL(__earlycon_table) = .;  \
>                          KEEP(*(__earlycon_table))              \
>                          VMLINUX_SYMBOL(__earlycon_table_end) = .;
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index b32df49a3bd5..c4219b9cbb70 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -351,10 +351,10 @@ struct earlycon_id {
>         char    name[16];
>         char    compatible[128];
>         int     (*setup)(struct earlycon_device *, const char *options);
> -} __aligned(32);
> +};
>
> -extern const struct earlycon_id __earlycon_table[];
> -extern const struct earlycon_id __earlycon_table_end[];
> +extern const struct earlycon_id *__earlycon_table[];
> +extern const struct earlycon_id *__earlycon_table_end[];
>
>  #if defined(CONFIG_SERIAL_EARLYCON) && !defined(MODULE)
>  #define EARLYCON_USED_OR_UNUSED        __used
> @@ -362,12 +362,19 @@ extern const struct earlycon_id __earlycon_table_end[];
>  #define EARLYCON_USED_OR_UNUSED        __maybe_unused
>  #endif
>
> -#define OF_EARLYCON_DECLARE(_name, compat, fn)                         \
> -       static const struct earlycon_id __UNIQUE_ID(__earlycon_##_name) \
> -            EARLYCON_USED_OR_UNUSED __section(__earlycon_table)        \
> +#define _OF_EARLYCON_DECLARE(_name, compat, fn, unique_id)             \
> +       static const struct earlycon_id unique_id                       \
> +            EARLYCON_USED_OR_UNUSED __initconst                        \
>                 = { .name = __stringify(_name),                         \
>                     .compatible = compat,                               \
> -                   .setup = fn  }
> +                   .setup = fn  };                                     \
> +       static const struct earlycon_id EARLYCON_USED_OR_UNUSED         \
> +               __section(__earlycon_table)                             \
> +               * const __PASTE(__p, unique_id) = &unique_id
> +
> +#define OF_EARLYCON_DECLARE(_name, compat, fn)                         \
> +       _OF_EARLYCON_DECLARE(_name, compat, fn,                         \
> +                            __UNIQUE_ID(__earlycon_##_name))
>
>  #define EARLYCON_DECLARE(_name, fn)    OF_EARLYCON_DECLARE(_name, "", fn)
>
> --
> 2.16.2.804.g6dcf76e118-goog
>