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SubjectRe: [PATCH v2 2/2] earlycon: Use a pointer table to fix __earlycon_table stride
On Tue, Mar 20, 2018 at 10:57 AM, Daniel Kurtz <djkurtz@chromium.org> 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

correspond

> to the 64-byte alignbed contents of section "__earlycon_table".

aligned

>
> 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@chromium.org>
> Suggested-by: Aaron Durbin <adurbin@chromium.org>
> ---
> Changes since v1:
> * added Suggested-by and Fixes, and reworded the commit message per Randy.
>
> 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..93b7add47087 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 __initdata \
> = { .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
>

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