On Fri, Feb 26, 2021 at 9:16 AM 'Pratyush Yadav' via Clang Built Linux
[off-list ref] wrote:

Hi,

On 25/02/21 01:55PM, Arnd Bergmann wrote:

quoted

From: Arnd Bergmann <arnd@arndb.de>

Building this file with clang leads to a an unreachable code path
causing a warning from objtool:

drivers/spi/spi-rockchip.o: warning: objtool: rockchip_spi_transfer_one()+0x2e0: sibling call from callable instruction with modified stack frame

Use BUG() instead of unreachable() to avoid the undefined behavior
if it does happen.

Fixes: 65498c6ae241 ("spi: rockchip: support 4bit words")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/spi/spi-rockchip.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/spi/spi-rockchip.c b/drivers/spi/spi-rockchip.c
index 936ef54e0903..972beac1169a 100644
--- a/drivers/spi/spi-rockchip.c
+++ b/drivers/spi/spi-rockchip.c

@@ -521,7 +521,7 @@ static void rockchip_spi_config(struct rockchip_spi *rs,
               * ctlr->bits_per_word_mask, so this shouldn't
               * happen
               */
-             unreachable();
+             BUG();

checkpatch says:

  Avoid crashing the kernel - try using WARN_ON & recovery code rather
  than BUG() or BUG_ON()

Which makes sense to me. This is not something bad enough to justify
crashing the kernel.

I thought about rewriting it more thoroughly when I wrote the patch, but
couldn't come up with a good alternative, so I did the simplest change
in this direction, replacing the silent crash with a loud one.

Should we just dev_warn() and return instead, hoping that it won't do
more harm?

The backtrace from WARN_ON() probably doesn't help here.

       Arnd

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