Re: [patch 2.6.26-rc3] spi: make spi_board_info.modalias a char array

On Wednesday 21 May 2008, Andrew Morton wrote:

quoted

On Wed, 21 May 2008 17:04:56 -0700 David Brownell [off-list ref] wrote:

quoted

From: Grant Likely <redacted>
            ...
-   proxy->modalias = chip->modalias;
+   strncpy(proxy->modalias, chip->modalias, KOBJ_NAME_LEN);
            ...

a) strncpy() doesn't null-terminate the dest if it overran.  strlcpy() does.

b) Given the uncertainly over the state of existing code, perhaps we
   should have an explicit check for overflows here, with a WARN_ON()?

c) I think it's better to use sizeof() in the strlcpy() rather than
   duplicating the array size - it's a little more robust in the face
   of future changes and it is more obviously-correct (don't need to go
   elsewhere to check the size of the destination).

Good points.  I usually try to use sizeof() myself, for exactly
that reason.  Updated version (below) uses strlcpy, sizeof, WARN_ON.

quoted

d) KOBJ_NAME_LEN no longer exists in linux-next.  I'm not sure where
   it went - Greg and Kay have been up to their usual tricks.

Yeech.  Replacing symbols with inline constants isn't good when
the constant is what ensures various fields are the same size.

BTW -- four comments on one line is pretty good even for you.  ;)

- Dave


========= CUT HERE
From: Grant Likely <redacted>

Currently, 'modalias' in the spi_device structure is a 'const char *'.
The spi_new_device() function fills in the modalias value from a passed
in spi_board_info data block.  Since it is a pointer copy, the new
spi_device remains dependent on the spi_board_info structure after the
new spi_device is registered (no other fields in spi_device directly
depend on the spi_board_info structure; all of the other data is copied).

This causes a problem when dynamically propulating the list of attached
SPI devices.  For example, in arch/powerpc, the list of SPI devices can
be populated from data in the device tree.  With the current code, the
device tree adapter must kmalloc() a new spi_board_info structure for
each new SPI device it finds in the device tree, and there is no simple
mechanism in place for keeping track of these allocations.

This patch changes modalias from a 'const char *' to a fixed char array.
By copying the modalias string instead of referencing it, the dependency
on the spi_board_info structure is eliminated and an outside caller does
not need to maintain a separate spi_board_info allocation for each device.

If searched through the code to the best of my ability for any references
to modalias which may be affected by this change and haven't found anything.
It has been tested with the lite5200b platform in arch/powerpc.

Signed-off-by: Grant Likely <redacted>
[ cope with linux-next changes: KOBJ_NAME_LEN obliterated, etc ]
Signed-off-by: David Brownell <redacted>
---
 drivers/spi/spi.c       |    4 +++-
 include/linux/spi/spi.h |    2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

--- a/drivers/spi/spi.c 2008-05-22 10:57:21.000000000 -0700
+++ b/drivers/spi/spi.c 2008-05-22 18:56:16.000000000 -0700

@@ -218,6 +218,8 @@ struct spi_device *spi_new_device(struct
       if (!spi_master_get(master))
               return NULL;

+       WARN_ON(strlen(chip->modalias) >= sizeof(proxy->modalias));
+
       proxy = kzalloc(sizeof *proxy, GFP_KERNEL);
       if (!proxy) {
               dev_err(dev, "can't alloc dev for cs%d\n",

@@ -229,7 +231,7 @@ struct spi_device *spi_new_device(struct
       proxy->max_speed_hz = chip->max_speed_hz;
       proxy->mode = chip->mode;
       proxy->irq = chip->irq;
-       proxy->modalias = chip->modalias;
+       strlcpy(proxy->modalias, chip->modalias, sizeof(proxy->modalias));

       snprintf(proxy->dev.bus_id, sizeof proxy->dev.bus_id,
                       "%s.%u", master->dev.bus_id,
--- a/include/linux/spi/spi.h   2008-05-22 10:57:21.000000000 -0700
+++ b/include/linux/spi/spi.h   2008-05-23 10:21:41.000000000 -0700

@@ -82,7 +82,7 @@ struct spi_device {
       int                     irq;
       void                    *controller_state;
       void                    *controller_data;
-       const char              *modalias;
+       char                    modalias[32];

       /*
        * likely need more hooks for more protocol options affecting how