[Qemu-devel] [PATCH] scsi: esp: clean up handle_ti/esp_do_dma if s->do_cmd

[Qemu-devel] [PATCH] scsi: esp: clean up handle_ti/esp_do_dma if s->do_cmd

[Paolo Bonzini]

Avoid duplicated code between esp_do_dma and handle_ti.  esp_do_dma
has the same code that handle_ti contains after the call to esp_do_dma;
but the code in handle_ti is never reached because it is in an "else if".
Remove the else and also the pointless return.

esp_do_dma also has a partially dead assignment of the to_device
variable.  Sink it to the point where it's actually used.

Finally, assert that the other caller of esp_do_dma (esp_transfer_data)
only transfers data and not a command.  This is true because get_cmd
cancels the old request synchronously before its caller handle_satn_stop
sets do_cmd to 1.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/scsi/esp.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 3f08598..2bc5076 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -245,21 +245,17 @@ static void esp_do_dma(ESPState *s)
     uint32_t len;
     int to_device;
 
-    to_device = (s->ti_size < 0);
     len = s->dma_left;
     if (s->do_cmd) {
         trace_esp_do_dma(s->cmdlen, len);
         s->dma_memory_read(s->dma_opaque, &s->cmdbuf[s->cmdlen], len);
-        s->ti_size = 0;
-        s->cmdlen = 0;
-        s->do_cmd = 0;
-        do_cmd(s, s->cmdbuf);
         return;
     }
     if (s->async_len == 0) {
         /* Defer until data is available.  */
         return;
     }
+    to_device = (s->ti_size < 0);
     if (len > s->async_len) {
         len = s->async_len;
     }
@@ -318,6 +314,7 @@ void esp_transfer_data(SCSIRequest *req, uint32_t len)
 {
     ESPState *s = req->hba_private;
 
+    assert(!s->do_cmd);
     trace_esp_transfer_data(s->dma_left, s->ti_size);
     s->async_len = len;
     s->async_buf = scsi_req_get_buf(req);
@@ -358,13 +355,13 @@ static void handle_ti(ESPState *s)
         s->dma_left = minlen;
         s->rregs[ESP_RSTAT] &= ~STAT_TC;
         esp_do_dma(s);
-    } else if (s->do_cmd) {
+    }
+    if (s->do_cmd) {
         trace_esp_handle_ti_cmd(s->cmdlen);
         s->ti_size = 0;
         s->cmdlen = 0;
         s->do_cmd = 0;
         do_cmd(s, s->cmdbuf);
-        return;
     }
 }
 
-- 
2.5.5

Re: [Qemu-devel] [PATCH] scsi: esp: clean up handle_ti/esp_do_dma if s->do_cmd

[Laszlo Ersek]

On 06/15/16 14:39, Paolo Bonzini wrote:
> Avoid duplicated code between esp_do_dma and handle_ti.  esp_do_dma
> has the same code that handle_ti contains after the call to esp_do_dma;
> but the code in handle_ti is never reached

(... never reached after esp_do_dma() is called -- it is reached in
general... but splitting hairs about this is not important)

> because it is in an "else if".
> Remove the else and also the pointless return.

Yes, this looks correct.

> esp_do_dma also has a partially dead assignment of the to_device
> variable.  Sink it to the point where it's actually used.

You could sink it a bit more, I think, to just before the first use.

> Finally, assert that the other caller of esp_do_dma (esp_transfer_data)
> only transfers data and not a command.  This is true because get_cmd
> cancels the old request synchronously before its caller handle_satn_stop
> sets do_cmd to 1.

I didn't try to verify why the claim is true, but if the claim is true,
then the assert() is valid, and fits in with the changes in esp_do_dma()
and handle_ti() -- the logic taken over by handle_ti() from esp_do_dma()
is not reached when esp_do_dma() is called by esp_transfer_data(), but
then again, on that call path, the original logic was never reached anyway.

(If the claim is wrong, we'll quickly find out with the assert() :))

... So, I think if you wish, you could lower the "to_device" assignment
a bit more. Either way:

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

And I guess Prasad will submit a new version of the buffer overflow fix,
on top of this patch, according to your previous message
<http://thread.gmane.org/gmane.comp.emulators.qemu/419113/focus=419155>.

Thanks
Laszlo

> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/scsi/esp.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
> index 3f08598..2bc5076 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -245,21 +245,17 @@ static void esp_do_dma(ESPState *s)
>      uint32_t len;
>      int to_device;
>  
> -    to_device = (s->ti_size < 0);
>      len = s->dma_left;
>      if (s->do_cmd) {
>          trace_esp_do_dma(s->cmdlen, len);
>          s->dma_memory_read(s->dma_opaque, &s->cmdbuf[s->cmdlen], len);
> -        s->ti_size = 0;
> -        s->cmdlen = 0;
> -        s->do_cmd = 0;
> -        do_cmd(s, s->cmdbuf);
>          return;
>      }
>      if (s->async_len == 0) {
>          /* Defer until data is available.  */
>          return;
>      }
> +    to_device = (s->ti_size < 0);
>      if (len > s->async_len) {
>          len = s->async_len;
>      }
> @@ -318,6 +314,7 @@ void esp_transfer_data(SCSIRequest *req, uint32_t len)
>  {
>      ESPState *s = req->hba_private;
>  
> +    assert(!s->do_cmd);
>      trace_esp_transfer_data(s->dma_left, s->ti_size);
>      s->async_len = len;
>      s->async_buf = scsi_req_get_buf(req);
> @@ -358,13 +355,13 @@ static void handle_ti(ESPState *s)
>          s->dma_left = minlen;
>          s->rregs[ESP_RSTAT] &= ~STAT_TC;
>          esp_do_dma(s);
> -    } else if (s->do_cmd) {
> +    }
> +    if (s->do_cmd) {
>          trace_esp_handle_ti_cmd(s->cmdlen);
>          s->ti_size = 0;
>          s->cmdlen = 0;
>          s->do_cmd = 0;
>          do_cmd(s, s->cmdbuf);
> -        return;
>      }
>  }
>  
> 

Re: [Qemu-devel] [PATCH] scsi: esp: clean up handle_ti/esp_do_dma if s->do_cmd

[Paolo Bonzini]

On 15/06/2016 15:16, Laszlo Ersek wrote:
> On 06/15/16 14:39, Paolo Bonzini wrote:
>> Finally, assert that the other caller of esp_do_dma (esp_transfer_data)
>> only transfers data and not a command.  This is true because get_cmd
>> cancels the old request synchronously before its caller handle_satn_stop
>> sets do_cmd to 1.
> 
> I didn't try to verify why the claim is true, but if the claim is true,
> then the assert() is valid, and fits in with the changes in esp_do_dma()
> and handle_ti() -- the logic taken over by handle_ti() from esp_do_dma()
> is not reached when esp_do_dma() is called by esp_transfer_data(), but
> then again, on that call path, the original logic was never reached anyway.
> 
> (If the claim is wrong, we'll quickly find out with the assert() :))

Correct. :)

> ... So, I think if you wish, you could lower the "to_device" assignment
> a bit more.

Will do, thanks.

Paolo

Re: [Qemu-devel] [PATCH] scsi: esp: clean up handle_ti/esp_do_dma if s->do_cmd

[P J P]

+-- On Wed, 15 Jun 2016, Laszlo Ersek wrote --+
| And I guess Prasad will submit a new version of the buffer overflow fix,
| on top of this patch, according to your previous message
| <http://thread.gmane.org/gmane.comp.emulators.qemu/419113/focus=419155>.

Yes, I'm preparing an update.
--
Prasad J Pandit / Red Hat Product Security Team
47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F