Re: [PATCH v2] tracing/dma: Cap dma_map_sg tracepoint arrays to prevent buffer overflow

[Sean Anderson <sean.anderson@linux.dev>]

On 1/30/26 10:52, Deepanshu Kartikey wrote:
> The dma_map_sg tracepoint can trigger a perf buffer overflow when
> tracing large scatter-gather lists. With devices like virtio-gpu
> creating large DRM buffers, nents can exceed 1000 entries, resulting
> in:
> 
>   phys_addrs: 1000 * 8 bytes = 8,000 bytes
>   dma_addrs:  1000 * 8 bytes = 8,000 bytes
>   lengths:    1000 * 4 bytes = 4,000 bytes
>   Total: ~20,000 bytes
> 
> This exceeds PERF_MAX_TRACE_SIZE (8192 bytes), causing:
> 
>   WARNING: CPU: 0 PID: 5497 at kernel/trace/trace_event_perf.c:405
>   perf buffer not large enough, wanted 24620, have 8192
> 
> Cap all three dynamic arrays at 128 entries using min() in the array
> size calculation. This ensures arrays are only as large as needed
> (up to the cap), avoiding unnecessary memory allocation for small
> operations while preventing overflow for large ones.
> 
> The tracepoint now records the full nents/ents counts and a truncated
> flag so users can see when data has been capped.
> 
> Changes in v2:
> - Use min(nents, DMA_TRACE_MAX_ENTRIES) for dynamic array sizing
>   instead of fixed DMA_TRACE_MAX_ENTRIES allocation (feedback from
>   Steven Rostedt)
> - This allocates only what's needed up to the cap, avoiding waste
>   for small operations
> 
> Reported-by: syzbot+28cea38c382fd15e751a@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=28cea38c382fd15e751a
> Tested-by: syzbot+28cea38c382fd15e751a@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
> ---
>  include/trace/events/dma.h | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
> index b3fef140ae15..33e99e792f1a 100644
> --- a/include/trace/events/dma.h
> +++ b/include/trace/events/dma.h
> @@ -275,6 +275,8 @@ TRACE_EVENT(dma_free_sgt,
>  				sizeof(u64), sizeof(u64)))
>  );
>  
> +#define DMA_TRACE_MAX_ENTRIES 128
> +
>  TRACE_EVENT(dma_map_sg,
>  	TP_PROTO(struct device *dev, struct scatterlist *sgl, int nents,
>  		 int ents, enum dma_data_direction dir, unsigned long attrs),
> @@ -282,9 +284,12 @@ TRACE_EVENT(dma_map_sg,
>  
>  	TP_STRUCT__entry(
>  		__string(device, dev_name(dev))
> -		__dynamic_array(u64, phys_addrs, nents)
> -		__dynamic_array(u64, dma_addrs, ents)
> -		__dynamic_array(unsigned int, lengths, ents)
> +		__field(int, full_nents)
> +		__field(int, full_ents)
> +		__field(bool, truncated)
> +		__dynamic_array(u64, phys_addrs,  min(nents, DMA_TRACE_MAX_ENTRIES))
> +		__dynamic_array(u64, dma_addrs, min(ents, DMA_TRACE_MAX_ENTRIES))
> +		__dynamic_array(unsigned int, lengths, min(ents, DMA_TRACE_MAX_ENTRIES))
>  		__field(enum dma_data_direction, dir)
>  		__field(unsigned long, attrs)
>  	),
> @@ -292,11 +297,16 @@ TRACE_EVENT(dma_map_sg,
>  	TP_fast_assign(
>  		struct scatterlist *sg;
>  		int i;
> +		int traced_nents = min_t(int, nents, DMA_TRACE_MAX_ENTRIES);
> +		int traced_ents = min_t(int, ents, DMA_TRACE_MAX_ENTRIES);
>  
>  		__assign_str(device);
> -		for_each_sg(sgl, sg, nents, i)
> +		__entry->full_nents = nents;
> +		__entry->full_ents = ents;
> +		__entry->truncated = (nents > DMA_TRACE_MAX_ENTRIES) || (ents > DMA_TRACE_MAX_ENTRIES);
> +		for_each_sg(sgl, sg, traced_nents, i)
>  			((u64 *)__get_dynamic_array(phys_addrs))[i] = sg_phys(sg);
> -		for_each_sg(sgl, sg, ents, i) {
> +		for_each_sg(sgl, sg, traced_ents, i) {
>  			((u64 *)__get_dynamic_array(dma_addrs))[i] =
>  				sg_dma_address(sg);
>  			((unsigned int *)__get_dynamic_array(lengths))[i] =
> @@ -306,9 +316,12 @@ TRACE_EVENT(dma_map_sg,
>  		__entry->attrs = attrs;
>  	),
>  
> -	TP_printk("%s dir=%s dma_addrs=%s sizes=%s phys_addrs=%s attrs=%s",
> +	TP_printk("%s dir=%s nents=%d/%d ents=%d/%d%s dma_addrs=%s sizes=%s phys_addrs=%s attrs=%s",
>  		__get_str(device),
>  		decode_dma_data_direction(__entry->dir),
> +		min_t(int, __entry->full_nents, DMA_TRACE_MAX_ENTRIES), __entry->full_nents,
> +		min_t(int, __entry->full_ents, DMA_TRACE_MAX_ENTRIES), __entry->full_ents,
> +		__entry->truncated ? " [TRUNCATED]" : "",
>  		__print_array(__get_dynamic_array(dma_addrs),
>  			      __get_dynamic_array_len(dma_addrs) /
>  				sizeof(u64), sizeof(u64)),

Reviwed-by: Sean Anderson <sean.anderson@linux.dev>

Although it's a bit unusual that there's no limit on dynamic arrays like there is for %*ph.