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![Chris Wilson](/assets/img/avatar_default.png)
An idea for extending uABI inspired by Vulkan's extension chains. Instead of expanding the data struct for each ioctl every time we need to add a new feature, define an extension chain instead. As we add optional interfaces to control the ioctl, we define a new extension struct that can be linked into the ioctl data only when required by the user. The key advantage being able to ignore large control structs for optional interfaces/extensions, while being able to process them in a consistent manner. In comparison to other extensible ioctls, the key difference is the use of a linked chain of extension structs vs an array of tagged pointers. For example, struct drm_amdgpu_cs_chunk { __u32 chunk_id; __u32 length_dw; __u64 chunk_data; }; struct drm_amdgpu_cs_in { __u32 ctx_id; __u32 bo_list_handle; __u32 num_chunks; __u32 _pad; __u64 chunks; }; allows userspace to pass in array of pointers to extension structs, but must therefore keep constructing that array along side the command stream. In dynamic situations like that, a linked list is preferred and does not similar from extra cache line misses as the extension structs themselves must still be loaded separate to the chunks array. v2: Apply the tail call optimisation directly to nip the worry of stack overflow in the bud. v3: Defend against recursion. v4: Fixup local types to match new uabi Opens: - do we include the result as an out-field in each chain? struct i915_user_extension { __u64 next_extension; __u64 name; __s32 result; __u32 mbz; /* reserved for future use */ }; * Undecided, so provision some room for future expansion. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190322092325.5883-1-chris@chris-wilson.co.uk
61 lines
1.1 KiB
C
61 lines
1.1 KiB
C
/*
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* SPDX-License-Identifier: MIT
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*
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* Copyright © 2018 Intel Corporation
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*/
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#include <linux/nospec.h>
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#include <linux/sched/signal.h>
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#include <linux/uaccess.h>
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#include <uapi/drm/i915_drm.h>
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#include "i915_user_extensions.h"
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#include "i915_utils.h"
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int i915_user_extensions(struct i915_user_extension __user *ext,
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const i915_user_extension_fn *tbl,
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unsigned int count,
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void *data)
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{
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unsigned int stackdepth = 512;
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while (ext) {
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int i, err;
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u32 name;
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u64 next;
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if (!stackdepth--) /* recursion vs useful flexibility */
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return -E2BIG;
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err = check_user_mbz(&ext->flags);
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if (err)
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return err;
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for (i = 0; i < ARRAY_SIZE(ext->rsvd); i++) {
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err = check_user_mbz(&ext->rsvd[i]);
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if (err)
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return err;
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}
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if (get_user(name, &ext->name))
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return -EFAULT;
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err = -EINVAL;
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if (name < count) {
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name = array_index_nospec(name, count);
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if (tbl[name])
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err = tbl[name](ext, data);
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}
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if (err)
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return err;
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if (get_user(next, &ext->next_extension) ||
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overflows_type(next, ext))
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return -EFAULT;
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ext = u64_to_user_ptr(next);
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}
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return 0;
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}
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