Re: [Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Pooja Dhannawat]

On Mon, Mar 14, 2016 at 7:41 PM, Eric Blake  wrote:

> On 03/12/2016 01:39 AM, Pooja Dhannawat wrote:
> > Signed-off-by: Pooja Dhannawat 
> > ---
> >  net/net.c | 8 ++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/net.c b/net/net.c
> > index b0c832e..5399758 100644
> > --- a/net/net.c
> > +++ b/net/net.c
> > @@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc,
> const uint8_t *buf, int size)
> >  static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec
> *iov,
> > int iovcnt, unsigned flags)
> >  {
> > -uint8_t buf[NET_BUFSIZE];
> > +uint8_t *buf;
> >  uint8_t *buffer;
> >  size_t offset;
> >
> > +buf = g_new(uint8_t, 1);
>
> If you're only going to malloc() one byte, it's more efficient to just
> stack-allocate it:
>
> uint8_t buf[1];
>
> Did you mean to do:
>
> buf = g_new(uint8_t, NET_BUFSIZE)
>
> instead?
>
> Yes, my bad, I meant NET_BUFSIZE.

> > +
> >  if (iovcnt == 1) {
> >  buffer = iov[0].iov_base;
> >  offset = iov[0].iov_len;
> >  } else {
> >  buffer = buf;
> > -offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
> > +offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf,
> sizeof(uint8_t));
>
> This is wrong.  The old code used NET_BUFSIZE bytes for iov_to_buf(),
> the new code uses only one byte.  By the way, sizeof(char) and
> sizeof(uint8_t) are pointless in code, as they are strictly equivalent to
> 1.
>
> I agree that the idea behind the patch is useful (NET_BUFSIZE is 68k,
> which is too large for our goal of at most 4k stack allocation if we
> never want to overflow a stack guard page), but you'll need a correct
> working version of the patch.
>
>   Yes, I will correct it and send next version of patch.

> --
> Eric Blake   eblake redhat com+1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>
>

Re: [Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Eric Blake]

On 03/12/2016 01:39 AM, Pooja Dhannawat wrote:
> Signed-off-by: Pooja Dhannawat 
> ---
>  net/net.c | 8 ++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/net.c b/net/net.c
> index b0c832e..5399758 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const 
> uint8_t *buf, int size)
>  static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
> int iovcnt, unsigned flags)
>  {
> -uint8_t buf[NET_BUFSIZE];
> +uint8_t *buf;
>  uint8_t *buffer;
>  size_t offset;
>  
> +buf = g_new(uint8_t, 1);

If you're only going to malloc() one byte, it's more efficient to just
stack-allocate it:

uint8_t buf[1];

Did you mean to do:

buf = g_new(uint8_t, NET_BUFSIZE)

instead?

> +
>  if (iovcnt == 1) {
>  buffer = iov[0].iov_base;
>  offset = iov[0].iov_len;
>  } else {
>  buffer = buf;
> -offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
> +offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, sizeof(uint8_t));

This is wrong.  The old code used NET_BUFSIZE bytes for iov_to_buf(),
the new code uses only one byte.  By the way, sizeof(char) and
sizeof(uint8_t) are pointless in code, as they are strictly equivalent to 1.

I agree that the idea behind the patch is useful (NET_BUFSIZE is 68k,
which is too large for our goal of at most 4k stack allocation if we
never want to overflow a stack guard page), but you'll need a correct
working version of the patch.

-- 
Eric Blake   eblake redhat com+1-919-301-3266
Libvirt virtualization library http://libvirt.org



signature.asc
Description: OpenPGP digital signature

Re: [Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Li Zhijian]

could you explain why we need this change ?


On 03/12/2016 04:39 PM, Pooja Dhannawat wrote:

Signed-off-by: Pooja Dhannawat 
---
  net/net.c | 8 ++--
  1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/net.c b/net/net.c
index b0c832e..5399758 100644
--- a/net/net.c
+++ b/net/net.c
@@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const 
uint8_t *buf, int size)
  static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
 int iovcnt, unsigned flags)
  {
-uint8_t buf[NET_BUFSIZE];
+uint8_t *buf;
  uint8_t *buffer;
  size_t offset;

+buf = g_new(uint8_t, 1);
+
  if (iovcnt == 1) {
  buffer = iov[0].iov_base;
  offset = iov[0].iov_len;
  } else {
  buffer = buf;
-offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
+offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, sizeof(uint8_t));
  }

  if (flags & QEMU_NET_PACKET_FLAG_RAW && nc->info->receive_raw) {
@@ -726,6 +728,8 @@ static ssize_t nc_sendv_compat(NetClientState *nc, const 
struct iovec *iov,
  } else {

forget to release 'buf'


  return nc->info->receive(nc, buffer, offset);
  }
+
+g_free(buf);
  }

  ssize_t qemu_deliver_packet_iov(NetClientState *sender,



--
Best regards.
Li Zhijian

[Qemu-devel] [Patch 1/1] net/net: Allocating Large sized arrays to heap

[Pooja Dhannawat]

Signed-off-by: Pooja Dhannawat 
---
 net/net.c | 8 ++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/net.c b/net/net.c
index b0c832e..5399758 100644
--- a/net/net.c
+++ b/net/net.c
@@ -709,16 +709,18 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const 
uint8_t *buf, int size)
 static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
int iovcnt, unsigned flags)
 {
-uint8_t buf[NET_BUFSIZE];
+uint8_t *buf;
 uint8_t *buffer;
 size_t offset;
 
+buf = g_new(uint8_t, 1);
+
 if (iovcnt == 1) {
 buffer = iov[0].iov_base;
 offset = iov[0].iov_len;
 } else {
 buffer = buf;
-offset = iov_to_buf(iov, iovcnt, 0, buf, sizeof(buf));
+offset = iov_to_buf(iov, iovcnt, 0, (uint8_t *)buf, sizeof(uint8_t));
 }
 
 if (flags & QEMU_NET_PACKET_FLAG_RAW && nc->info->receive_raw) {
@@ -726,6 +728,8 @@ static ssize_t nc_sendv_compat(NetClientState *nc, const 
struct iovec *iov,
 } else {
 return nc->info->receive(nc, buffer, offset);
 }
+
+g_free(buf);
 }
 
 ssize_t qemu_deliver_packet_iov(NetClientState *sender,
-- 
2.5.0