Why does call_usermodehelper fail most of the times?

Question

From a kernel module, I am trying to use call_usermodehelper function to execute an executable sha1 which takes a file as argument and writes the SHA1 hash sum of the file t

Answer 1

The last argument for call_usermodehelper is actually some sort of enumeration:

#define UMH_NO_WAIT     0       /* don't wait at all */
#define UMH_WAIT_EXEC   1       /* wait for the exec, but not the process */
#define UMH_WAIT_PROC   2       /* wait for the process to complete */
#define UMH_KILLABLE    4       /* wait for EXEC/PROC killable */

As you can see, with wait=1 the function waits while exec is performed, but doesn't wait the process.

If no other constraints, value UMH_WAIT_PROC gives more stable results.