Xoscope nuisance console messages on Pi4 running -current

[Mark Millard]

On 2021-May-2, at 23:37, bob prohaska <fbsd at www.zefox.net> wrote:

> After a successful compile of audio/xoscope on a Pi4 running current a
> stream of messages appeared on the console and in the security log
> while xoscope was running:
> 
> 
> +WARNING pid 26370 (xoscope): ioctl sign-extension ioctl ffffffffc0045006
> +WARNING pid 26370 (xoscope): ioctl sign-extension ioctl ffffffffc0045005
> +WARNING pid 26370 (xoscope): ioctl sign-extension ioctl ffffffffc0045002
> +WARNING pid 26370 (xoscope): ioctl sign-extension ioctl ffffffffc0045006
> +WARNING pid 26370 (xoscope): ioctl sign-extension ioctl ffffffffc0045005
> +WARNING pid 26370 (xoscope): ioctl sign-extension ioctl ffffffffc0045002
> 
> They seem to come at a fairly high rate and clutter the logfiles, but 
> apart from that nuisance nothing else seemed visibly amiss.
> 
> Are they of any significance?

The code from a mid 2021-Mar vintage of main [so: 14]
looks like:

/* ARGSUSED */
int
sys_ioctl(struct thread *td, struct ioctl_args *uap)
{
        u_char smalldata[SYS_IOCTL_SMALL_SIZE] __aligned(SYS_IOCTL_SMALL_ALIGN);
        uint32_t com;
        int arg, error;
        u_int size;
        caddr_t data;

#ifdef INVARIANTS
        if (uap->com > 0xffffffff) {
                printf(
                    "WARNING pid %d (%s): ioctl sign-extension ioctl %lx\n",
                    td->td_proc->p_pid, td->td_name, uap->com);
        }
#endif
        com = (uint32_t)uap->com;
. . .

where sys/sys/sysproto.h shows:

struct ioctl_args {
        char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)];
        char com_l_[PADL_(u_long)]; u_long com; char com_r_[PADR_(u_long)];
        char data_l_[PADL_(char *)]; char * data; char data_r_[PADR_(char *)];
};

So, uap->com is unsigned with 64 bits for aarch64 and the
0xffffffff is converted to match that for the > test (C
rules).

The assignment to uint32_t com would end up with a
truncated value recorded whenever the warning is
produced.

Thus a value like 0xffffffffc0045006ul ends up with
com == 0xc0045006u instead.

Then looking at the usage of com leads to sys/sys/ioccom.h :

. . .
/*
 * Ioctl's have the command encoded in the lower word, and the size of
 * any in or out parameters in the upper word.  The high 3 bits of the
 * upper word are used to encode the in/out status of the parameter.
 *
 *       31 29 28                     16 15            8 7             0
 *      +---------------------------------------------------------------+
 *      | I/O | Parameter Length        | Command Group | Command       |
 *      +---------------------------------------------------------------+
 */
#define IOCPARM_SHIFT   13              /* number of bits for ioctl size */
#define IOCPARM_MASK    ((1 << IOCPARM_SHIFT) - 1) /* parameter length mask */
#define IOCPARM_LEN(x)  (((x) >> 16) & IOCPARM_MASK)
#define IOCBASECMD(x)   ((x) & ~(IOCPARM_MASK << 16))
#define IOCGROUP(x)     (((x) >> 8) & 0xff)

#define IOCPARM_MAX     (1 << IOCPARM_SHIFT) /* max size of ioctl */

#define IOC_VOID        0x20000000UL    /* no parameters */
#define IOC_OUT         0x40000000UL    /* copy out parameters */
#define IOC_IN          0x80000000UL    /* copy in parameters */
#define IOC_INOUT       (IOC_IN|IOC_OUT)/* copy parameters in and out */
#define IOC_DIRMASK     (IOC_VOID|IOC_OUT|IOC_IN)/* mask for IN/OUT/VOID */
. . .

So, com == 0xc0045006u ends up with:

I/O matching:                : IOC_IN | IOC_OUT
Command matching       (byte): 0x06u
Command Group matching (byte): 0x50u
Parameter Length matching    : 0x0004u

While I do not know the specifics for the command
and command group encoding, the truncated value
seems coherent with the code that is using it.

My guess would be xoscope used a signed 32-bit type
that got a value with sign extension to 64 bits
before the value started being treated as unsigned.
If it had used an unsigned type instead, the padding
would have been a zero fill instead (presuming that
I've guessed right).

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)