Coding bug in the function serial_ioport_write in serial.c

[Jonathan D. Belanger]

Branch hash: b50ea0d (pulled from github).

I was reviewing the code and noticed the following in the function serial_ioport_write:

    assert(size == 1 && addr < 8);
        .
        .
        .
    switch(addr) {
    default:
    case 0:
        if (s->lcf & UART_LCR_DLAB) {
            if (size == 1) {
                s->divider = (s->divider & 0xff00) | val;
            } else {
                s->divider = val;
            }
        }

The assert will trigger if the size is > 1, so the else of the if (size == 1) will never be executed and an attempt to specify a size > 1 will trigger an assert.

The documentation for the UART indicates that the 16-bit divisor is broken up amongst 2 8-bit registers (DLL and DLM). There already is code to handle the DLL and DLM portions of the divider register (as coded).

This is not exactly going to cause a bug, as there is no code that calls this function with a value for size other than 1. It is just unnecessary code.

[Philippe Mathieu-Daudé (philmd)]

Since commit 5ec3a23e6c8 ("serial: convert PIO to new memory
api read/write") we don't need to worry about accesses bigger
than 8-bit. Use the extract()/deposit() functions to access
the correct part of the 16-bit 'divider' register.

Reported-by: Jonathan D. Belanger <email address hidden>
Buglink: https://bugs.launchpad.net/qemu/+bug/1904331
Signed-off-by: Philippe Mathieu-Daudé <email address hidden>
---
Cc: Bug 1904331 <email address hidden>
---
 hw/char/serial.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/hw/char/serial.c b/hw/char/serial.c
index 97f71879ff2..62c627f486f 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -24,6 +24,7 @@
  */

 #include "qemu/osdep.h"
+#include "qemu/bitops.h"
 #include "hw/char/serial.h"
 #include "hw/irq.h"
 #include "migration/vmstate.h"
@@ -338,11 +339,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
     default:
     case 0:
         if (s->lcr & UART_LCR_DLAB) {
- if (size == 1) {
- s->divider = (s->divider & 0xff00) | val;
- } else {
- s->divider = val;
- }
+ s->divider = deposit32(s->divider, 8 * addr, 8, val);
             serial_update_parameters(s);
         } else {
             s->thr = (uint8_t) val;
@@ -364,7 +361,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
         break;
     case 1:
         if (s->lcr & UART_LCR_DLAB) {
- s->divider = (s->divider & 0x00ff) | (val << 8);
+ s->divider = deposit32(s->divider, 8 * addr, 8, val);
             serial_update_parameters(s);
         } else {
             uint8_t changed = (s->ier ^ val) & 0x0f;
@@ -478,7 +475,7 @@ static uint64_t serial_ioport_read(void *opaque, hwaddr addr, unsigned size)
     default:
     case 0:
         if (s->lcr & UART_LCR_DLAB) {
- ret = s->divider & 0xff;
+ ret = extract16(s->divider, 8 * addr, 8);
         } else {
             if(s->fcr & UART_FCR_FE) {
                 ret = fifo8_is_empty(&s->recv_fifo) ?
@@ -502,7 +499,7 @@ static uint64_t serial_ioport_read(void *opaque, hwaddr addr, unsigned size)
         break;
     case 1:
         if (s->lcr & UART_LCR_DLAB) {
- ret = (s->divider >> 8) & 0xff;
+ ret = extract16(s->divider, 8 * addr, 8);
         } else {
             ret = s->ier;
         }
--
2.26.2

[Thomas Huth (th-huth)]

https://gitlab.com/qemu-project/qemu/-/commit/29daa894b6c31eae074d