hw/ssi/mss-spi: Avoid crash when reading empty RX FIFO

Reading the RX_DATA register when the RX_FIFO is empty triggers
an abort. This can be easily reproduced:

  $ qemu-system-arm -M emcraft-sf2 -monitor stdio -S
  QEMU 4.0.50 monitor - type 'help' for more information
  (qemu) x 0x40001010
  Aborted (core dumped)

  (gdb) bt
  #1  0x00007f035874f895 in abort () at /lib64/libc.so.6
  #2  0x00005628686591ff in fifo8_pop (fifo=0x56286a9a4c68) at util/fifo8.c:66
  #3  0x00005628683e0b8e in fifo32_pop (fifo=0x56286a9a4c68) at include/qemu/fifo32.h:137
  #4  0x00005628683e0efb in spi_read (opaque=0x56286a9a4850, addr=4, size=4) at hw/ssi/mss-spi.c:168
  #5  0x0000562867f96801 in memory_region_read_accessor (mr=0x56286a9a4b60, addr=16, value=0x7ffeecb0c5c8, size=4, shift=0, mask=4294967295, attrs=...) at memory.c:439
  #6  0x0000562867f96cdb in access_with_adjusted_size (addr=16, value=0x7ffeecb0c5c8, size=4, access_size_min=1, access_size_max=4, access_fn=0x562867f967c3 <memory_region_read_accessor>, mr=0x56286a9a4b60, attrs=...) at memory.c:569
  #7  0x0000562867f99940 in memory_region_dispatch_read1 (mr=0x56286a9a4b60, addr=16, pval=0x7ffeecb0c5c8, size=4, attrs=...) at memory.c:1420
  #8  0x0000562867f99a08 in memory_region_dispatch_read (mr=0x56286a9a4b60, addr=16, pval=0x7ffeecb0c5c8, size=4, attrs=...) at memory.c:1447
  #9  0x0000562867f38721 in flatview_read_continue (fv=0x56286aec6360, addr=1073745936, attrs=..., buf=0x7ffeecb0c7c0 "\340ǰ\354\376\177", len=4, addr1=16, l=4, mr=0x56286a9a4b60) at exec.c:3385
  #10 0x0000562867f38874 in flatview_read (fv=0x56286aec6360, addr=1073745936, attrs=..., buf=0x7ffeecb0c7c0 "\340ǰ\354\376\177", len=4) at exec.c:3423
  #11 0x0000562867f388ea in address_space_read_full (as=0x56286aa3e890, addr=1073745936, attrs=..., buf=0x7ffeecb0c7c0 "\340ǰ\354\376\177", len=4) at exec.c:3436
  #12 0x0000562867f389c5 in address_space_rw (as=0x56286aa3e890, addr=1073745936, attrs=..., buf=0x7ffeecb0c7c0 "\340ǰ\354\376\177", len=4, is_write=false) at exec.c:3466
  #13 0x0000562867f3bdd7 in cpu_memory_rw_debug (cpu=0x56286aa19d00, addr=1073745936, buf=0x7ffeecb0c7c0 "\340ǰ\354\376\177", len=4, is_write=0) at exec.c:3976
  #14 0x000056286811ed51 in memory_dump (mon=0x56286a8c32d0, count=1, format=120, wsize=4, addr=1073745936, is_physical=0) at monitor/misc.c:730
  #15 0x000056286811eff1 in hmp_memory_dump (mon=0x56286a8c32d0, qdict=0x56286b15c400) at monitor/misc.c:785
  #16 0x00005628684740ee in handle_hmp_command (mon=0x56286a8c32d0, cmdline=0x56286a8caeb2 "0x40001010") at monitor/hmp.c:1082

From the datasheet "Actel SmartFusion Microcontroller Subsystem
User's Guide" Rev.1, Table 13-3 "SPI Register Summary", this
register has a reset value of 0.

Check the FIFO is not empty before accessing it, else log an
error message.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20190709113715.7761-3-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit c0bccee)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/ssi/mss-spi.c

@@ -164,7 +164,13 @@ spi_read(void *opaque, hwaddr addr, unsigned int size)
     case R_SPI_RX:
         s->regs[R_SPI_STATUS] &= ~S_RXFIFOFUL;
         s->regs[R_SPI_STATUS] &= ~S_RXCHOVRF;
-        ret = fifo32_pop(&s->rx_fifo);
+        if (fifo32_is_empty(&s->rx_fifo)) {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "%s: Reading empty RX_FIFO\n",
+                          __func__);
+        } else {
+            ret = fifo32_pop(&s->rx_fifo);
+        }
         if (fifo32_is_empty(&s->rx_fifo)) {
             s->regs[R_SPI_STATUS] |= S_RXFIFOEMP;
         }