Re: [PATCH v2] earlycon: Fix __earlycon_table stride... again

From: Daniel Kurtz
Date: Thu Mar 15 2018 - 12:15:57 EST

On Wed, Mar 14, 2018 at 8:33 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:

> On 03/14/2018 06:48 PM, Daniel Kurtz wrote:
> > Commit 470ca0de69fe ("serial: earlycon: Enable earlycon without command
> > line param") added EARLYCON_TABLE().
> >
> > Commit 99492c39f39f ("earlycon: Fix __earlycon_table stride") referenced
> > commit 07fca0e57fca92 ("tracing: Properly align linker defined symbols")
> > and tried to fix EARLYCON_TABLE() using __aligned(32).
> >
> > However, the fix copied just works around compiler behavior. In fact,
it
> > was subsequently re-implemented for tracing using a table of pointers in
> > commit 3d56e331b653 ("tracing: Replace syscall_meta_data struct array
with
> > pointer array"), commit 654986462939 ("tracepoints: Fix section
alignment
> > using pointer array") and commit e4a9ea5ee7c8 ("tracing: Replace
> > trace_event struct array with pointer array").
> >
> > Let's do the same "array of pointers to structs" approach for
> > EARLYCON_TABLE.

> Please describe what the problem is instead of just what the fix is.
> Thanks.

The problem is the same as in 99492c39f39f; the __earlycon_table stride may
be wrong depending on how the compiler decides to align the contents of
__earlycon_table.
See the other referenced patches for a detailed description of the problem
and its solution.

-Dan


> > Signed-off-by: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
> > ---
> > Changes since v1:
> > * Change __earlycon_table to an array of pointers.
> >
> > Note: I do not have a convenient system to test the of/fdt.c part of
this
> > patch, so would apreciate help verifying it. Thanks!
> >
> > 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 100644ï
> > --- 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)
> >
> >


> --
> ~Randy