KHR_robustness
Name
KHR_robustness
Name Strings
GL_KHR_robustness
Contributors
Daniel Koch, NVIDIA
Nicolas Capens, TransGaming
Contributors to ARB_robustness
Contact
Jon Leech (oddhack 'at' sonic.net)
Greg Roth, NVIDIA (groth 'at' nvidia.com)
Notice
Copyright (c) 2012-2014 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 and OpenGL ES Working Groups. 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 OpenGL ES Working Group on June 25, 2014.
Approved by the ARB on June 26, 2014.
Ratified by the Khronos Board of Promoters on August 7, 2014.
Version
Version 11, August 21, 2014
Number
ARB Extension #170
OpenGL ES Extension #190
Dependencies
OpenGL ES 2.0 or OpenGL 3.2 are required. Some features of this
extension are supported by OpenGL ES only if OpenGL ES 3.0 or later is
supported.
EGL_EXT_create_context_robustness is used to determine if a context
implementing this extension supports robust buffer access, and if it
supports reset notification.
This extension is written against the OpenGL ES 3.1 Specification
(version of June 4, 2014) and the OpenGL ES 3.10.3 Shading Language
Specification (version of June 6, 2014).
Overview
Several recent trends in how OpenGL ES integrates into modern computer
systems have created new requirements for robustness and security for GL
rendering contexts.
Additionally GPU architectures now support hardware fault detection;
for example, video memory supporting ECC (error correcting codes)
and error detection. GL contexts should be capable of recovering
from hardware faults such as uncorrectable memory errors. Along with
recovery from such hardware faults, the recovery mechanism can
also allow recovery from video memory access exceptions and system
software failures. System software failures can be due to device
changes or driver failures.
GL queries that return (write) some number of bytes to a
buffer indicated by a pointer parameter introduce risk of buffer
overflows that might be exploitable by malware. To address this,
queries with return value sizes that are not expressed directly by
the parameters to the query itself are given additional API
functions with an additional parameter that specifies the number of
bytes in the buffer and never writing bytes beyond that limit. This
is particularly useful for multi-threaded usage of GL contexts
in a "share group" where one context can change objects in ways that
can cause buffer overflows for another context's GL queries.
The original ARB_vertex_buffer_object extension includes an issue
that explicitly states program termination is allowed when
out-of-bounds vertex buffer object fetches occur. Modern graphics
hardware is capable of well-defined behavior in the case of out-of-
bounds vertex buffer object fetches. Older hardware may require
extra checks to enforce well-defined (and termination free)
behavior, but this expense is warranted when processing potentially
untrusted content.
The intent of this extension is to address some specific robustness
goals:
* For all existing GL queries, provide additional "safe" APIs
that limit data written to user pointers to a buffer size in
bytes that is an explicit additional parameter of the query.
* Provide a mechanism for a GL application to learn about
graphics resets that affect the context. When a graphics reset
occurs, the GL context becomes unusable and the application
must create a new context to continue operation. Detecting a
graphics reset happens through an inexpensive query.
* Define behavior of OpenGL calls made after a graphics reset.
* Provide an enable to guarantee that out-of-bounds buffer object
accesses by the GPU will have deterministic behavior and preclude
application instability or termination due to an incorrect buffer
access. Such accesses include vertex buffer fetches of
attributes and indices, and indexed reads of uniforms or
parameters from buffers.
New Procedures and Functions
NOTE: when implemented in an OpenGL ES context, all entry points defined
by this extension must have a "KHR" suffix. When implemented in an
OpenGL context, all entry points must have NO suffix, as shown below.
enum GetGraphicsResetStatus();
void ReadnPixels(int x, int y, sizei width, sizei height,
enum format, enum type, sizei bufSize,
void *data);
void GetnUniformfv(uint program, int location, sizei bufSize,
float *params);
void GetnUniformiv(uint program, int location, sizei bufSize,
int *params);
void GetnUniformuiv(uint program, int location, sizei bufSize,
uint *params);
New Tokens
NOTE: when implemented in an OpenGL ES context, all tokens defined by
this extension must have a "_KHR" suffix. When implemented in an OpenGL
context, all tokens must have NO suffix, as described below.
Returned by GetGraphicsResetStatus:
NO_ERROR 0x0000
GUILTY_CONTEXT_RESET 0x8253
INNOCENT_CONTEXT_RESET 0x8254
UNKNOWN_CONTEXT_RESET 0x8255
Accepted by the <value> parameter of GetBooleanv, GetIntegerv,
and GetFloatv:
CONTEXT_ROBUST_ACCESS 0x90F3
RESET_NOTIFICATION_STRATEGY 0x8256
Returned by GetIntegerv and related simple queries when <value> is
RESET_NOTIFICATION_STRATEGY :
LOSE_CONTEXT_ON_RESET 0x8252
NO_RESET_NOTIFICATION 0x8261
Returned by GetError:
CONTEXT_LOST 0x0507
Additions to the OpenGL ES 3.1 Specification
Add to section 2.3.1 "Errors" in the bullet list of implicit errors for
GL commands on p. 14, and modify the following paragraph:
* If the GL context has been reset as a result of a previous GL
command, or if the context is reset as a side effect of execution of
a command, a CONTEXT_LOST error is generated.
The Specification attempts to explicitly describe these implicit error
conditions (with the exception of OUT_OF_MEMORY [fn2] and CONTEXT_LOST
[fn3]) wherever they apply. However ...
[fn3] CONTEXT_LOST is not described because it occurs for reasons not
directly related to the affected commands, and applies to almost all GL
commands.
Add to table 2.3 "Summary of GL errors" on p. 15:
Error Description Offending command ignored?
------------ ----------------------- --------------------------
CONTEXT_LOST Context has been lost Except as noted for
and reset by the driver specific commands
Add a new subsection 2.3.1rob after 2.3.1 "GL Errors", and renumber
subsequent sections accordingly:
2.3.1rob "Graphics Reset Recovery"
Certain events can result in a reset of the GL context. After such an
event, it is referred to as a <lost context> and is unusable for almost
all purposes. Recovery requires creating a new context and recreating
all relevant state from the lost context. The current status of the
graphics reset state is returned by
enum GetGraphicsResetStatus();
The value returned indicates if the GL context has been in a reset state
at any point since the last call to GetGraphicsResetStatus:
* NO_ERROR indicates that the GL context has not been in a reset state
since the last call.
* GUILTY_CONTEXT_RESET indicates that a reset has been detected
that is attributable to the current GL context.
* INNOCENT_CONTEXT_RESET indicates a reset has been detected that
is not attributable to the current GL context.
* UNKNOWN_CONTEXT_RESET indicates a detected graphics reset whose
cause is unknown.
If a reset status other than NO_ERROR is returned and subsequent calls
return NO_ERROR, the context reset was encountered and completed. If a
reset status is repeatedly returned, the context may be in the process
of resetting.
Reset notification behavior is determined at context creation time, and
may be queried by calling GetIntegerv with the symbolic constant
RESET_NOTIFICATION_STRATEGY.
If the reset notification behavior is NO_RESET_NOTIFICATION, then
the implementation will never deliver notification of reset events, and
GetGraphicsResetStatus will always return NO_ERROR[fn1].
[fn1: In this case it is recommended that implementations should not
allow loss of context state no matter what events occur. However,
this is only a recommendation, and cannot be relied upon by
applications.]
If the behavior is LOSE_CONTEXT_ON_RESET, a graphics reset will
result in a lost context and require creating a new context as described
above. In this case GetGraphicsResetStatus will return an appropriate
value from those described above.
If a graphics reset notification occurs in a context, a notification
must also occur in all other contexts which share objects with that
context[fn2].
[fn2: The values returned by GetGraphicsResetStatus in the
different contexts may differ.]
After a graphics reset has occurred on a context, subsequent GL commands
on that context (or any context which shares with that context) will
generate a CONTEXT_LOST error. Such commands will not have side effects
(in particular, they will not modify memory passed by pointer for query
results), and may not block indefinitely or cause termination of the
application. Exceptions to this behavior include:
* GetError and GetGraphicsResetStatus behave normally following a
graphics reset, so that the application can determine a reset has
occurred, and when it is safe to destroy and recreate the context.
* Any commands which might cause a polling application to block
indefinitely will generate a CONTEXT_LOST error, but will also
return a value indicating completion to the application. Such
commands include:
+ GetSynciv with <pname> SYNC_STATUS ignores the other parameters
and returns SIGNALED in <values>.
+ GetQueryObjectuiv with <pname> QUERY_RESULT_AVAILABLE ignores the
other parameters and returns TRUE in <params>.
Modify section 7.12 "Shader, Program, and Program Pipeline Queries"
on p. 125 to add the GetnUniform* commands:
The commands
[enumerate existing GetUniform*v commands, then add]
void GetnUniformfv(uint program, int location, sizei bufSize,
float *params);
void GetnUniformiv(uint program, int location, sizei bufSize,
int *params);
void GetnUniformuiv(uint program, int location, sizei bufSize,
uint *params);
return the value or values of the uniform at location <location> of the
default uniform block for program object <program> in the array
<params>. The type of the uniform at <location> determines the number of
values returned. Calling GetnUniform*v ensures that no more than
<bufSize> bytes are written into <params>.
Add to the Errors section for Get*Uniform* on p. 126:
An INVALID_OPERATION error is generated by GetnUniform* if the buffer
size required to store the requested data is greater than <bufSize>
Add subsection 10.3.1.1rob prior to section 10.3.2 "Vertex Attribute
Divisors" on p. 241:
10.3.1.1rob Robust Buffer Access Behavior
Robust buffer access is enabled by creating a context with robust access
enabled through the window system binding APIs. When enabled, indices
within the element array that reference vertex data that lies outside
the enabled attribute's vertex buffer object
[for OpenGL ES] result in undefined values
[for OpenGL] result in reading zero
for the corresponding attributes, but cannot result in application
failure.
Robust buffer access behavior may be queried by calling
GetIntegerv with the symbolic constant CONTEXT_ROBUST_ACCESS.
Modify section 16.1.2 "ReadPixels" on p. 332 to add the ReadnPixels
command:
Pixels are read using
void ReadPixels(int x, int y, sizei width, sizei height,
enum format, enum type, void *data);
void ReadnPixels(int x, int y, sizei width, sizei height,
enum format, enum type, sizei bufSize,
void *data);
The arguments after <x> and <y> to ReadPixels ... are summarized in
table 16.1. ReadnPixels behaves identically to ReadPixels except that
it does not write more than <bufSize> bytes into <data>.
Add to the Errors section for ReadPixels and ReadnPixels:
An INVALID_OPERATION error is generated by ReadnPixels if the buffer
size required to store the requested data is greater than <bufSize>.
Additions to The OpenGL ES Shading Language Specification, Version 3.10
Modify the first paragraph of section 4.1.9 "Arrays" on p. 30:
Variables of the same type ... Undefined behavior results from indexing
an array with a non-constant expression that is greater than or equal to
the array size or less than 0. If robust buffer access is enabled (see
section 10.3.1.1rob of the OpenGL ES 3.1 API Specification), such
indexing must not result in abnormal program termination. The results
are still undefined, but implementations are encouraged to produce zero
values for such accesses. Arrays only have a single dimension ...
Dependencies on OpenGL ES
For implementations against OpenGL ES, if OpenGL ES 3.0 or a later
version is not supported, remove all references to GetnUniformuiv and
remove the exceptional behavior of GetSynciv and GetQueryObjectuiv for
lost contexts.
Interactions with EGL_EXT_create_context_robustness
If the EGL window-system binding API is used to create a context, the
EGL_EXT_create_context_robustness extension is supported, and the
attribute EGL_CONTEXT_OPENGL_ROBUST_ACCESS_EXT is set to EGL_TRUE when
eglCreateContext is called, the resulting context will perform robust
buffer access as described above in section 10.3.1.1rob, and the
CONTEXT_ROBUST_ACCESS query will return GL_TRUE.
If the EGL window-system binding API is used to create a context and the
EGL_EXT_create_context_robustness extension is supported, then the value
of attribute EGL_CONTEXT_RESET_NOTIFICATION_STRATEGY_EXT determines the
reset notification behavior and the value of
RESET_NOTIFICATION_STRATEGY, as described in section 2.3.1rob.
New Implementation Dependent State
Get Value Type Get Command Minimum Value Description Sec. Attribute
--------------------------- ---- ----------- ----------------- --------------------------- ----------- ---------
CONTEXT_ROBUST_ACCESS B GetIntegerv - Robust access enabled 10.3.1.1rob -
RESET_NOTIFICATION_STRATEGY Z_2 GetIntegerv See sec. 2.3.1rob Reset notification behavior 2.3.1rob -
Issues
(Issues 2-8 are identical to those in the base EXT_robustness
extension).
1. What should this extension be called?
RESOLVED: KHR_robustness
This is just the OpenGL ES EXT_robustness extension (itself based on
ARB_robustness) promoted to KHR status, with consistency with OpenGL
4.5 behavior and phrasing included.
2. How does this extension differ from Desktop GL's ARB_robustness?
RESOLVED: Because EGL_EXT_create_context_robustness uses a separate
attribute to enable robust buffer access, a corresponding query is
added here.
Also see issue 12.
3. Should we provide a context creation mechanism to enable this
extension?
RESOLVED. Yes.
Currently, EGL_EXT_create_context_robustness provides this mechanism
via two unique attributes. These attributes differ from those
specified by KHR_create_context to allow for differences in what
functionality those attributes define.
4. What can cause a graphics reset?
Either user or implementor errors may result in a graphics reset. If
the application attempts to perform a rendering that takes too long
whether due to an infinite loop in a shader or even just a rendering
operation that takes too long on the given hardware. Implementation
errors may produce badly formed hardware commands. Memory access
errors may result from user or implementor mistakes. On some systems,
power management events such as system sleep, screen saver
activation, or pre-emption may also context resets to occur. Any of
these events may result in a graphics reset event that will be
detectable by the mechanism described in this extension.
5. How should the application react to a reset context event?
RESOLVED: For this extension, the application is expected to query
the reset status until NO_ERROR is returned. If a reset is
encountered, at least one *RESET* status will be returned. Once
NO_ERROR is again encountered, the application can safely destroy the
old context and create a new one.
After a reset event, apps should not use a context for any purpose
other than determining its reset status (using either GetError to
identify a CONTEXT_LOST error, or GetGraphicsResetStatus()), and
then destroying it. If a context receives a reset event, all other
contexts in its share group will also receive reset events, and
should be destroyed and recreated.
Apps should be cautious in interpreting the GUILTY and INNOCENT reset
statuses. These are guidelines to the immediate cause of a reset, but
not guarantees of the ultimate cause.
6. If a graphics reset occurs in a shared context, what happens in
shared contexts?
RESOLVED: A reset in one context will result in a reset in all other
contexts in its share group.
7. How can an application query for robust buffer access support, since
this is now determined at context creation time?
RESOLVED. The application can query the value of
CONTEXT_ROBUST_ACCESS using GetIntegerv. If true, this
functionality is enabled.
8. How is the reset notification behavior controlled?
RESOLVED: Reset notification behavior is determined at context
creation time using EGL/GLX/WGL/etc. mechanisms. In order that shared
objects be handled predictably, a context cannot share with another
context unless both have the same reset notification behavior.
9. How does this extension differ from EXT_robustness?
RESOLVED: By adding the new CONTEXT_LOST error, removing support for
ES 1.1 (for logistical reasons), and changing suffixes from EXT to
KHR (for ES) or no suffix (for GL).
-
How should this extension be enabled via EGL?
PROPOSED: Either by using EGL 1.5 context creation APIs (see section 3.7.1.5 of the EGL 1.5 spec), or EGL_EXT_create_context_robustness. This interaction will be noted in both the EGL 1.5 spec and the extension spec by allowing at least one, and possibly both of GL_EXT_robustness and GL_KHR_robustness to be supported by contexts created via these mechanisms. If a context supporting GL_KHR_robustness is created, it will optionally support GL_KHR_robust_buffer_access_behavior as well (see issue 5 of that extension).
-
What should the behavior of GL calls made after a context is lost be? This can be expected to occur when the context has been lost, but the app hasn't polled the reset status yet.
RESOLVED: Set a new CONTEXT_LOST error on all GL calls (except GetError and GetGraphicsReset).
DISCUSSION: GetError and GetGraphicsResetStatus must continue to work at least to the extent of their interactions with robustness features, so that apps can determine a context was lost and see the effect on other commands. Commands which might block indefinitely or cause the app to block indefinitely while polling are defined to return immediately, with values which should end a polling loop. Such commands include sync and query object queries for completion.
Special handling of these commands is defined to return values which should not cause blocking, but also to generate the CONTEXT_LOST error. This is intended to deal well both with apps that are polling on a query value, and apps that may test for the error immediately after each command (such as debuggers). There has been no pushback against
We believe there are no other commands requiring this special handling in the core API. ClientWaitSync and WaitSync were also proposed, but with the current language (which specifies that commands on a lost context may not block) should not need exceptional handling.
REJECTED OPTIONS:
A) GL will try to handle the call as best as possible. Depending on the call this may use GPU resources which may now have undefined contents.
B) GL calls become no-ops. This would be like having a null-dispatch table installed when you don't have a current context. The problem with this approach is that any call that has return parameters won't fill them in and the application won't be able to detect that a value was not returned.
REJECTED SUB-FEATURES:
We discussed using OUT_OF_MEMORY or INVALID_OPERATION. Both are misleading and don't uniquely identify the problem as a runtime error out of scope of anything the app did.
We discussed allowing all commands to have side effects in addition to generating CONTEXT_LOST, such as putting a "safe" value into return parameters. Without compelling reason to allow this behavior, it is cleaner to define the spec to leave results unchanged aside from the exceptional cases intended to prevent hanging on polling loops.
-
What changed in promoting OES_robustness to KHR_robustness? What remains to be done for consistency between GL and ES?
DISCUSSION: The ARB agreed to support robustness and robust_buffer_access_behavior as a KHR extension, with the following chnages and issues to be resolved during the ratification period (none are IP-related):
a) As was done for KHR_debug, the extension is defined to have KHR suffixes only in ES implementations. GL implementations do not have suffixes. This is so KHR_robustness can be used as a backwards-compatibility extension for OpenGL 4.5.
b) Minor spec language tweaks were done for consistency with OpenGL 4.5, to eliminate redundancy. None are functional with the exception of a change in section 10.3.1.1rob, which imposes stronger requirements on out-of-bounds indexed attribute access for GL (returns zero) relative to ES (undefined return values).
c) The ARB wants to add these commands to the extension, as defined in OpenGL 4.5. They would be only be supported in GL contexts:
GetnUniformdv GetnTexImage GetnCompressedTexImageWe do not think we need to add the compatibility-mode Getn* queries defined by ARB_robustness.
d) OpenGL 4.5 exposes support for robustness by setting CONTEXT_FLAG_ROBUST_ACCESS_BIT in the CONTEXT_FLAGS query. ES (and this extension) expose it with the CONTEXT_ROBUST_ACCESS query. Jon proposes we resolve this by only defining the CONTEXT_FLAGS query for GL, but defining the CONTEXT_ROBUST_ACCESS query for both GL and ES, and aliasing the flag bit with the access boolean. This will result in minor changes to both GL 4.5 and this extension.
e) This extension modifies the Shading Language specification in section 4.1.9 to restrict behavior of out of bounds array accesses. GLSL 4.50 has a considerably broader section 5.11 "Out-of-Bounds Accesses" which restricts behavior of arrays, vectors, structs, etc. We will want to include that language in KHR_robustness at least as GLSL-specific language; if ES wants to adopt the broader language for GLSL-ES that might still be doable.
Revision History
Rev. Date Author Changes
---- ------------ --------- ------------------------------------------
11 2014/08/21 Jon Leech Fix typo.
10 2014/06/26 Jon Leech Change from OES to KHR. Update issues 1
and 9; add issue 12 on ES / GL
differences.
9 2014/06/26 Jon Leech Minor phrasing fixes to be more consistent
with equivalent GL functionality.
8 2014/06/24 Jon Leech Fix typos & mention GetError in issue 5.
Reorder API spec edits in section order.
Resolve issues 9-11 (Bug 12104 comments
#42-45). Assign CONTEXT_LOST enum.
7 2014/06/02 Jon Leech Remove "safe" return value behavior for
queries when contexts are lost (Bug 12104
comment #27).
6 2014/06/02 Jon Leech Rebase extension on OpenGL ES 3.1 specs
and reflow paragraphs (no functionality
changes are made in this version).
5 2014/05/16 Jon Leech Add CONTEXT_LOST error and exceptional
behavior for potentially-blocking
commands (Bug 8411).
4 2014/05/14 Jon Leech Add issue 11 on behavior of GL calls made
after a context is lost.
3 2014/05/07 Jon Leech Add GetnUniformuivOES for ES 3.0.
2 2014/04/26 Jon Leech Updates based on Ken's comments in bug
12104 and to reference EGL 1.5.
1 2014/04/23 Jon Leech Branch from EXT_robustness version 3 and
promote to OES suffixes. Add issues 9-10.