| Name |
| |
| ARB_robust_buffer_access_behavior |
| |
| Name Strings |
| |
| GL_ARB_robust_buffer_access_behavior |
| |
| Contact |
| |
| Piers Daniell, NVIDIA (pdaniell 'at' nvidia.com) |
| |
| Contributors |
| |
| Jan-Harald Fredriksen, AMD |
| Jeff Bolz, NVIDIA |
| Kenneth Russell, Google |
| Pat Brown, NVIDIA |
| |
| Notice |
| |
| Copyright (c) 2012-2013 The Khronos Group Inc. Copyright terms at |
| http://www.khronos.org/registry/speccopyright.html |
| |
| Specification Update Policy |
| |
| Khronos-approved extension specifications are updated in response to |
| issues and bugs prioritized by the Khronos OpenGL Working Group. For |
| extensions which have been promoted to a core Specification, fixes will |
| first appear in the latest version of that core Specification, and will |
| eventually be backported to the extension document. This policy is |
| described in more detail at |
| https://www.khronos.org/registry/OpenGL/docs/update_policy.php |
| |
| Status |
| |
| Complete. |
| Approved by the ARB on 2012/06/12. |
| |
| Version |
| |
| Last Modified Date: October 2, 2013 |
| Version: 10 |
| |
| Number |
| |
| ARB Extension #135 |
| |
| Dependencies |
| |
| The ARB_robustness extension is required. |
| |
| This extension is written against The OpenGL 4.2 (Compatibility Profile) |
| specification. |
| |
| Overview |
| |
| This extension specifies the behavior of out-of-bounds buffer and array |
| accesses. This is an improvement over the existing ARB_robustness |
| extension which stated that the application should not crash, but |
| the behavior is otherwise undefined. This extension specifies the access |
| protection provided by the GL to ensure that out-of-bounds accesses |
| cannot read from or write to data not owned by the application. All |
| accesses are contained within the buffer object and program area they |
| reference. These additional robustness guarantees apply to contexts |
| created with the CONTEXT_FLAG_ROBUST_ACCESS_BIT_ARB feature enabled. |
| |
| IP Status |
| |
| No known IP claims. |
| |
| New Procedures and Functions |
| |
| None |
| |
| New Tokens |
| |
| None |
| |
| Additions to Chapter 2 of the OpenGL 4.2 (Compatibility Profile) |
| Specification (OpenGL Operation) |
| |
| Add to Section 2.8 "Vertex Arrays" before 2.8.1 (Packed Vertex Data |
| Formats): |
| |
| "Robust buffer access can be enabled by creating a context with robust |
| access enabled through the window system binding APIs. When enabled, |
| indices within the elements array that reference vertex data that |
| lies outside the enabled attributes vertex buffer objects, result in |
| reading zero. It is not possible to read vertex data from outside the |
| enabled vertex buffer objects or from another GL context, and these |
| accesses do not result in abnormal program termination." |
| |
| Add to Section 2.9.4 (Effects of Accessing Outside Buffer Bounds) on |
| page 65 the following: |
| |
| "Robust buffer access can be enabled by creating a context with robust |
| access enabled through the window system binding APIs. When enabled, any |
| command unable to set a GL error as described above, for example buffer |
| object accesses from the active program, will not read or modify memory |
| outside of the buffer object and will not result in GL interruption or |
| termination. Out-of-bounds reads may return values from within the buffer |
| object or zero. Out-of-bounds writes may modify values within the buffer |
| object or be discarded. Accesses made through resources attached to |
| binding points are only protected within the buffer object from which the |
| binding point is declared. For example, for an out-of-bounds access to |
| a member variable of a uniform block, the access protection is provided |
| within the uniform buffer object, and not for the bound buffer range for |
| this uniform block." |
| |
| Add a new paragraph to the end of the "Texel Fetches" sub-section on page |
| 147 that reads: |
| |
| "In all the above cases, if the context was created with robust buffer |
| access enabled then instead of undefined behavior the result of the |
| texel fetch is zero. For the case of a texel fetch on an incomplete |
| texture the result is a texture source color of (0, 0, 0, 1)." |
| |
| Add the following paragraph to the sub-section titled "Undefined Behavior" |
| near the bottom of page 152: |
| |
| "Robust buffer access can be enabled by creating a context with robust |
| access enabled through the window system binding APIs. When enabled, |
| out-of-bounds accesses will be bounded within the working memory of the |
| active program and cannot access memory owned by other GL contexts, |
| and will not result in abnormal program termination. Out-of-bounds |
| access to local and global variables cannot read values from other |
| program invocations. An out-of-bounds read may return another value |
| from the active program's working memory or zero. An out-of-bounds |
| write may overwrite a value from the active program's working memory |
| or be discarded. |
| |
| Out-of-bounds accesses to resources backed by buffer objects cannot |
| read or modify data outside of the buffer object. For resources bound |
| to buffer ranges, access is restricted within the buffer object from |
| which the buffer range was created, and not within the buffer range |
| itself. Out-of-bounds reads may return values from within the buffer |
| object or zero. Out-of-bounds writes may modify values within the buffer |
| object or be discarded. |
| |
| Out-of-bounds accesses to arrays of resources, such as an array of |
| textures, can only access the data of bound resources. Reads from |
| unbound resources return zero and writes are discarded. It is not |
| possible to access data owned by other GL contexts. |
| |
| Applications that require defined behavior for out-of-bounds accesses |
| should range check all computed indices before dereferencing the array, |
| vector or matrix." |
| |
| Additions to The OpenGL Shading Language Specification, Version 4.20.6 |
| |
| Add a new Section 5.11 (Out-of-Bounds Access and Robust Buffer Access |
| Behavior) |
| |
| "In the sub-sections described above for array, vector, matrix and |
| structure accesses, any out-of-bounds access produced undefined behavior. |
| However, if robust buffer access is enabled via the OpenGL API, such accesses will |
| be bound within the memory extent of the active program. It will not be |
| possible to access memory from other programs, and accesses will not result in |
| abnormal program termination. Out-of-bounds reads return undefined values, |
| which include values from other variables of the active program or zero. |
| Out-of-bounds writes may be discarded or overwrite other variables of the |
| active program, depending on the value of the computed index and how this |
| relates to the extent of the active program's memory. Applications that |
| require defined behavior for out-of-bounds accesses should range check |
| all computed indices before dereferencing an array." |
| |
| Errors |
| |
| None |
| |
| New State |
| |
| None |
| |
| New Implementation Dependent State |
| |
| None |
| |
| Issues |
| |
| 1) Why are out-of-bounds reads of buffer object backed resourced defined |
| to "return zero or a value from the current buffer object" rather than |
| just "return zero"? |
| |
| RESOLVED: This is necessary to allow the implementation to either |
| range check, apply a mask/modulus or apply a clamp to the index. |
| |
| 2) Why do out-of-bounds writes to buffer object backed resources have a |
| stronger guarantee than reads? The spec says that writes outside of |
| the bounded range are discarded but reads are only protected within |
| the buffer object. |
| |
| RESOLVED: This stronger guarentee for writes can be made because |
| GPUs made since around 2008 (DX10) already implement this behavior. |
| |
| Revision History |
| |
| Rev. Date Author Changes |
| ---- ------------ --------- ------------------------------------------- |
| 10 02 Oct 2013 Jon Leech Drop bogus clause about reading zero for |
| ``disabled attributes'' in section 2.8 |
| (Bug 10695). |
| |
| 9 15 May 2012 pdaniell Specify that texel fetches are defined to |
| return zero for the error cases only when |
| robust buffer access is enabled. |
| |
| 8 20 Apr 2012 pdaniell Mark issues 1 and 2 resolved. |
| |
| 7 15 Mar 2012 pdaniell Edits based on reviews from Kenneth Russel |
| and Jan-Harald Fredriksen. |
| |
| 6 30 Jan 2012 pdaniell Edits based on internal review. |
| |
| 5 24 Jan 2012 pdaniell Add spec language to define out of bounds |
| access behavior of vertex array index, |
| buffer objects and shader arrays, vectors |
| and matrices. |
| |
| 4 24 Jan 2012 pdaniell ARBify. Still need to convert to real spec. |
| |
| 3 28 Nov 2011 pdaniell Simplify the spec to better represent what |
| today's desktop GPUs already support. |
| |
| 2 16 Nov 2011 pdaniell Add some more detail to the overview |
| |
| 1 27 Oct 2011 kbr Initial version |
