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a=e1sVV491RgrpLwSTMOnk8w==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=FelO9ux0wxsA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=jiCTI4zE5U7BLdzWsZGv:22 a=3I1J8UUJPc9JN9BFgKH3:22 a=7CQSdrXTAAAA:8 a=VwQbUJbxAAAA:8 a=yPCof4ZbAAAA:8 a=fK1WjxYyLWqF990s5I4A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 a=a-qgeE7W1pNrGK8U0ZQC:22 a=5yU3S35YU4bGjq-dph-N:22 a=Bho9c0fBagfJEIQBS7DQ:22 cc=ntf awl=host:13723 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNjI5MDA4NyBTYWx0ZWRfX4VD692RWcdMR c4SGHIq3x0EN/9AXqyvJX3ajSmRYVThgl81u6w8HoQHKP4x/BYN57YgquTyDfJmhD5DpW0eWoDt LSpNgYEfM7Qnz/TO08YMFb4xFYF75chxG9O/apeObVOWnH0o5mfwWjSp/LcoMM5eI+9N6yqo3KO 4SdqQIS+N+EtNZmjMFkxgFKdFO2JkrcW8AbxcboZXIHGuiIjZAEF4F7T7nIumQP1T1S+jcwNMCq CJoFrj72Nb5ljd6Abnru1cPXeGeRAh/6yTGcWgtwShFMucNzwYh0iKuD99Z+m5zrIb8ynGYfTPX Osh8rRlArPbQoL/lLJ394xMa+bwlT4jU4NBuTomFccdWEyTyBGSlSmvo0LmDA6uuyzN957xIkb0 l9UzZ1IvsC/xdd0Jz6hoAp1KPzH+64Qwd6alvZKl6W9QOfiq3AG8re1K90ZZfRoUtesEAb4CipJ Hn646VPSrVOu9SdoSat1NKzV94HgKEHWOHfu38LE= X-Proofpoint-Spam-Info: AW1haW4tMjYwNjI5MDA4NyBTYWx0ZWRfX7tt9FkOeWc3v U1PiRVIe7MyzVCazJjx7Cui9LSjLfvqy7PXH49dp/yyRelL7VAc4NbNPJ8mrNj2KB892yNsdzN2 Yq6KmYBI7Jmo8Fdk6AIpROplcMnwqYoRDqIKsOo3Dc8q+ZuWSiEH On 29/06/2026 11:11, Robin Murphy wrote: > On 29/06/2026 9:53 am, John Garry wrote: >> For when a device is not DMA capable, the max mapping size would be 0, so >> make dma_max_mapping_size() reflect that. > > Seems logical. > > Reviewed-by: Robin Murphy Thanks, but sashiko has been reviewing this and has a few things to say. I don't know how it chooses recipients. On 29/06/2026 10:10, sashiko-bot@kernel.org wrote: >> >> + if (!dev->dma_mask) >> + return 0; > [Severity: High] > Could this new return value of 0 cause a division by zero in callers that > expect a strictly positive maximum mapping size? > > For instance, in drivers/accel/amdxdna/amdxdna_cbuf.c:amdxdna_cbuf_map(), > the returned size is used directly as a divisor: > > max_seg = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev)); > n_entries = (cbuf->node.size + max_seg - 1) / max_seg; Later in amdxdna_cbuf_map() we call dma_map_resource() (-> dma_map_phys()) and this would fail and WARN for !dev->dma_mask. Indeed amdxdna_cbuf_map() is used as a callback for .map_dma_buf, so highly unrealistic to have !dev->dma_mask ever. > > If a device lacking a dma_mask maps the attachment, max_seg becomes 0, > which would result in an immediate kernel panic. > > Additionally, could this cause invalid zero-sized block layer queue limits > in MMC drivers? In drivers/mmc/host/bcm2835.c:bcm2835_add_host(), the > return value clamps max_req_size: > > mmc->max_req_size = min_t(size_t, 524288, dma_max_mapping_size(dev)); bcm2835.c is a platform device driver, and platform devices have their dev->dma_mask set in setup_pdev_dma_masks() Indeed, that driver does have a non-DMA mode of operation, but that looks to be selected independent of whether dev->dma_mask is set. > > If the device lacks DMA support, max_req_size becomes 0, which propagates > to the block layer and can trigger panics when processing requests. All other users of dma_max_mapping_size() are drivers for real/virtio HW, so should be no issues. > >> Signed-off-by: John Garry >> --- >>   kernel/dma/mapping.c | 3 +++ >>   1 file changed, 3 insertions(+) >> >> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c >> index 4fe04669e5e66..7e576e5c6b8be 100644 >> --- a/kernel/dma/mapping.c >> +++ b/kernel/dma/mapping.c >> @@ -979,6 +979,9 @@ size_t dma_max_mapping_size(struct device *dev) >>       const struct dma_map_ops *ops = get_dma_ops(dev); >>       size_t size = SIZE_MAX; >> +    if (!dev->dma_mask) >> +        return 0; >> + >>       if (dma_map_direct(dev, ops)) >>           size = dma_direct_max_mapping_size(dev); >>       else if (use_dma_iommu(dev)) >