[RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

[RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

[Robert Mader]

As udmabuf increasingly enjoys popularity - being used in projects like
libcamera, Gstreamer, Mesa and KWin - users more frequently encounter
cases where the current default size limit of 64MB is too low. Examples
include allocating video buffers at a 8K resolution - and even 4K is
affected when using non-subsampled video formats or high bit depths.

While the limit can already be changed via the kernel command line,
exposing it as a kernel config makes that easier and more discoverable
for distros. Thus let's do that.

Signed-off-by: Robert Mader <robert.mader@collabora.com>
---
 drivers/dma-buf/Kconfig   | 6 ++++++
 drivers/dma-buf/udmabuf.c | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
index 7efc0f0d0712..35f0779cdc80 100644
--- a/drivers/dma-buf/Kconfig
+++ b/drivers/dma-buf/Kconfig
@@ -40,6 +40,12 @@ config UDMABUF
 	  A driver to let userspace turn memfd regions into dma-bufs.
 	  Qemu can use this to create host dmabufs for guest framebuffers.
 
+config UDMABUF_SIZE_LIMIT_MBYTES
+	int "Size limit in Mega Bytes"
+	default 64
+	help
+	  Maximum size of a udmabuf, in megabytes. Default is 64.
+
 config DMABUF_DEBUG
 	bool "DMA-BUF debug checks"
 	depends on DMA_SHARED_BUFFER
diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index bced421c0d65..a83153326362 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -20,7 +20,11 @@ static int list_limit = 1024;
 module_param(list_limit, int, 0644);
 MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024.");
 
+#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
+static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES;
+#else
 static int size_limit_mb = 64;
+#endif
 module_param(size_limit_mb, int, 0644);
 MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64.");
 
-- 
2.55.0

[RFC PATCH 2/2] dma-buf/udmabuf: Increase default size limit to 256MB

[Robert Mader]

While there does not seem to be a particularly strong reason to have a
size limit for udmabufs in the first place, raising the limit moderatly
to 256MB is enough to avoid hitting the limit in increasingly common
use-cases - notably allocating video buffers for 8K resolutions.

Signed-off-by: Robert Mader <robert.mader@collabora.com>
---
 drivers/dma-buf/Kconfig   | 4 ++--
 drivers/dma-buf/udmabuf.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
index 35f0779cdc80..0e7f5dc978e2 100644
--- a/drivers/dma-buf/Kconfig
+++ b/drivers/dma-buf/Kconfig
@@ -42,9 +42,9 @@ config UDMABUF
 
 config UDMABUF_SIZE_LIMIT_MBYTES
 	int "Size limit in Mega Bytes"
-	default 64
+	default 256
 	help
-	  Maximum size of a udmabuf, in megabytes. Default is 64.
+	  Maximum size of a udmabuf, in megabytes. Default is 256.
 
 config DMABUF_DEBUG
 	bool "DMA-BUF debug checks"
diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index a83153326362..c7649a04b865 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -23,10 +23,10 @@ MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024.
 #ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
 static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES;
 #else
-static int size_limit_mb = 64;
+static int size_limit_mb = 256;
 #endif
 module_param(size_limit_mb, int, 0644);
-MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64.");
+MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 256.");
 
 struct udmabuf {
 	pgoff_t pagecount;
-- 
2.55.0

Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:

New issues:
- [Low] CONFIG_UDMABUF_SIZE_LIMIT_MBYTES lacks a dependency on CONFIG_UDMABUF.
- [Low] Missing range constraint on UDMABUF_SIZE_LIMIT_MBYTES allows negative values, bypassing size validation.

Pre-existing issues:
- [High] Concurrent calls to DMA_BUF_IOCTL_SYNC trigger a race condition in begin_cpu_udmabuf, leading to a memory leak.
--

--- Patch [1]: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES ---
commit 258354f822e15700d93d45826a3e51b308dd35b8
Author: Robert Mader <robert.mader@collabora.com>

dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

This patch introduces a new Kconfig option, CONFIG_UDMABUF_SIZE_LIMIT_MBYTES,
to allow distributions to configure the maximum size limit for udmabufs at 
compile time. The limit defaults to 64MB and was previously only configurable 
via the kernel command line.

> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index 7efc0f0d07126..35f0779cdc801 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -40,6 +40,12 @@ config UDMABUF
>  	  A driver to let userspace turn memfd regions into dma-bufs.
>  	  Qemu can use this to create host dmabufs for guest framebuffers.
>  
> +config UDMABUF_SIZE_LIMIT_MBYTES
> +	int "Size limit in Mega Bytes"
> +	default 64

[Severity: Low]
Should this config have a 'depends on UDMABUF' directive?

Without it, users might be prompted to configure the size limit even
when the UDMABUF driver itself is disabled, polluting the global config.

[Severity: Low]
Could a user configure a negative value here and bypass the size limits?

If this is set to a negative value, it appears it will wrap around to a 
massive value when cast to u64 in udmabuf_create():

drivers/dma-buf/udmabuf.c:udmabuf_create() {
...
	pglimit = ((u64)size_limit_mb * 1024 * 1024) >> PAGE_SHIFT;
...
}

Would it be safer to add a range constraint to the Kconfig definition?

> +	help
> +	  Maximum size of a udmabuf, in megabytes. Default is 64.
> +
>  config DMABUF_DEBUG
>  	bool "DMA-BUF debug checks"
>  	depends on DMA_SHARED_BUFFER

[ ... ]

> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index bced421c0d658..a831533263621 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -20,7 +20,11 @@ static int list_limit = 1024;
>  module_param(list_limit, int, 0644);
>  MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024.");
>  
> +#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
> +static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES;
> +#else
>  static int size_limit_mb = 64;
> +#endif
>  module_param(size_limit_mb, int, 0644);
>  MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64.");
>  

[Severity: High]
This is a pre-existing issue, but can concurrent calls to 
DMA_BUF_IOCTL_SYNC cause a memory leak?

Looking at begin_cpu_udmabuf(), the scatter-gather table pointer is 
checked and assigned without locks:

drivers/dma-buf/udmabuf.c:begin_cpu_udmabuf() {
...
	if (!ubuf->sg) {
		ubuf->sg = get_sg_table(dev, buf, direction);
...
}

If multiple threads invoke the ioctl simultaneously, could they bypass 
the check, allocate multiple tables, and overwrite ubuf->sg, permanently
leaking the previously allocated tables and mappings?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260711144814.8205-1-robert.mader@collabora.com?part=1

Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

[Christian König]

On 7/11/26 16:48, Robert Mader wrote:
> As udmabuf increasingly enjoys popularity - being used in projects like
> libcamera, Gstreamer, Mesa and KWin - users more frequently encounter
> cases where the current default size limit of 64MB is too low. Examples
> include allocating video buffers at a 8K resolution - and even 4K is
> affected when using non-subsampled video formats or high bit depths.
> 
> While the limit can already be changed via the kernel command line,
> exposing it as a kernel config makes that easier and more discoverable
> for distros. Thus let's do that.

Well config options are usually only useful if the value can't be changed on runtime through a module parameter, but that is clearly not the case here.

On the other hand I do see your problem. I would just vote to disable the limit by default, there is nothing preventing userspace from allocating multiple uDMA-bufs so it doesn't seem to prevent any security issue or similar.

Regards,
Christian.


> 
> Signed-off-by: Robert Mader <robert.mader@collabora.com>
> ---
>  drivers/dma-buf/Kconfig   | 6 ++++++
>  drivers/dma-buf/udmabuf.c | 4 ++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index 7efc0f0d0712..35f0779cdc80 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -40,6 +40,12 @@ config UDMABUF
>           A driver to let userspace turn memfd regions into dma-bufs.
>           Qemu can use this to create host dmabufs for guest framebuffers.
> 
> +config UDMABUF_SIZE_LIMIT_MBYTES
> +       int "Size limit in Mega Bytes"
> +       default 64
> +       help
> +         Maximum size of a udmabuf, in megabytes. Default is 64.
> +
>  config DMABUF_DEBUG
>         bool "DMA-BUF debug checks"
>         depends on DMA_SHARED_BUFFER
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index bced421c0d65..a83153326362 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -20,7 +20,11 @@ static int list_limit = 1024;
>  module_param(list_limit, int, 0644);
>  MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024.");
> 
> +#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
> +static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES;
> +#else
>  static int size_limit_mb = 64;
> +#endif
>  module_param(size_limit_mb, int, 0644);
>  MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64.");
> 
> --
> 2.55.0
> 

Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

[Robert Mader]

Hi,

On 13.07.26 11:12, Christian König wrote:
> On 7/11/26 16:48, Robert Mader wrote:
>> As udmabuf increasingly enjoys popularity - being used in projects like
>> libcamera, Gstreamer, Mesa and KWin - users more frequently encounter
>> cases where the current default size limit of 64MB is too low. Examples
>> include allocating video buffers at a 8K resolution - and even 4K is
>> affected when using non-subsampled video formats or high bit depths.
>>
>> While the limit can already be changed via the kernel command line,
>> exposing it as a kernel config makes that easier and more discoverable
>> for distros. Thus let's do that.
> Well config options are usually only useful if the value can't be changed on runtime through a module parameter, but that is clearly not the case here.
>
> On the other hand I do see your problem. I would just vote to disable the limit by default, there is nothing preventing userspace from allocating multiple uDMA-bufs so it doesn't seem to prevent any security issue or similar.

I fully agree and would prefer that as well. If there is no resistance / 
concerns from anybody I'll send a corresponding patch at the end of the 
week.

Thanks,

Robert

>
> Regards,
> Christian.
>
>
>> Signed-off-by: Robert Mader <robert.mader@collabora.com>
>> ---
>>   drivers/dma-buf/Kconfig   | 6 ++++++
>>   drivers/dma-buf/udmabuf.c | 4 ++++
>>   2 files changed, 10 insertions(+)
>>
>> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
>> index 7efc0f0d0712..35f0779cdc80 100644
>> --- a/drivers/dma-buf/Kconfig
>> +++ b/drivers/dma-buf/Kconfig
>> @@ -40,6 +40,12 @@ config UDMABUF
>>            A driver to let userspace turn memfd regions into dma-bufs.
>>            Qemu can use this to create host dmabufs for guest framebuffers.
>>
>> +config UDMABUF_SIZE_LIMIT_MBYTES
>> +       int "Size limit in Mega Bytes"
>> +       default 64
>> +       help
>> +         Maximum size of a udmabuf, in megabytes. Default is 64.
>> +
>>   config DMABUF_DEBUG
>>          bool "DMA-BUF debug checks"
>>          depends on DMA_SHARED_BUFFER
>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>> index bced421c0d65..a83153326362 100644
>> --- a/drivers/dma-buf/udmabuf.c
>> +++ b/drivers/dma-buf/udmabuf.c
>> @@ -20,7 +20,11 @@ static int list_limit = 1024;
>>   module_param(list_limit, int, 0644);
>>   MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024.");
>>
>> +#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES
>> +static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES;
>> +#else
>>   static int size_limit_mb = 64;
>> +#endif
>>   module_param(size_limit_mb, int, 0644);
>>   MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64.");
>>
>> --
>> 2.55.0
>>