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object_get_property.md

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zx_object_get_property

SUMMARY

Ask for various properties of various kernel objects.

DECLARATION

#include <zircon/syscalls.h>

zx_status_t zx_object_get_property(zx_handle_t handle,
                                   uint32_t property,
                                   void* value,
                                   size_t value_size);

DESCRIPTION

zx_object_get_property() requests the value of a kernel object's property. Getting a property requires ZX_RIGHT_GET_PROPERTY rights on the handle.

The handle parameter indicates the target kernel object. Different properties only work on certain types of kernel objects, as described below.

The property parameter indicates which property to get/set. Property values have the prefix ZX_PROP_, and are described below.

The value parameter holds the property value, and must be a pointer to a buffer of value_size bytes. Different properties expect different value types/sizes as described below.

PROPERTIES

Property values have the prefix ZX_PROP_, and are defined in

#include <zircon/syscalls/object.h>

ZX_PROP_NAME

handle type: (Most types)

value type: char[ZX_MAX_NAME_LEN]

Allowed operations: get, set

The name of the object, as a NUL-terminated string.

ZX_PROP_REGISTER_FS and ZX_PROP_REGISTER_GS

handle type: Thread

value type: uintptr_t

Allowed operations: get, set

The value of the x86 FS.BASE or GS.BASE register, respectively. value must be a canonical address.

This is a software substitute for the rdfsbase, wrfsbase and rdgsbase, wrgsbase instruction pairs supported on newer x86-64 CPUs, and should behave exactly the same as using the CPU instructions directly (except that attempting to set a noncanonical address as the value just gets an error return rather than generating a machine exception). When using a CPU that supports these instructions (as reported by the cpuid instruction), it's more efficient and simpler to use the machine instructions directly.

Only defined for x86-64.

ZX_PROP_PROCESS_DEBUG_ADDR

handle type: Process

value type: uintptr_t

Allowed operations: get, set

The value of ld.so's _dl_debug_addr. This can be used by debuggers to interrogate the state of the dynamic loader.

If this value is set to ZX_PROCESS_DEBUG_ADDR_BREAK_ON_SET on process creation, the loader will manually issue a debug breakpoint when the property has been set to its correct value. This gives an opportunity to read or modify the initial state of the program.

ZX_PROP_PROCESS_BREAK_ON_LOAD

handle type: Process

value type: uintptr_t

Allowed operations: get, set

Determines whether the dynamic loader will issue a debug trap on every load of a shared library. If set before the first thread of a process runs, it will also trigger a debug trap for the initial load.

The dynamic loader sets the expected value of ZX_PROP_PROCESS_DEBUG_ADDR before triggering this debug trap. Exception handlers can use this property to query the dynamic loader's state.

When the dynamic loader issues the debug trap, it also sets the value of ZX_PROP_PROCESS_BREAK_ON_LOAD to the address of the debug trap, so that a debugger could compare the value with the address of the exception to determine whether the debug trap was triggered by the dynamic loader.

Any non-zero value is considered to activate this feature. Setting this property to zero will disable it. A debugger could also use this property to detect whether there's already another debugger attached to the same process.

Note: Depending on the architecture, the address reported by the exception might be different that the one reported by this property. For example, an x64 platform reports the instruction pointer after it executes the instruction. This means that an x64 platform reports an instruction pointer one byte higher than this property.

ZX_PROP_PROCESS_VDSO_BASE_ADDRESS

handle type: Process

value type: uintptr_t

Allowed operations: get

The base address of the vDSO mapping, or zero.

ZX_PROP_PROCESS_HW_TRACE_CONTEXT_ID

handle type: Process

value type: uintptr_t

Allowed operations: get

The context ID distinguishes different processes in hardware instruction tracing. On Intel X86-64 this is the value of register CR3.

To obtain ZX_PROP_PROCESS_HW_TRACE_CONTEXT_ID, you must specify kernel.enable-debugging-syscalls=true on the kernel command line. Otherwise, the function returns ZX_ERR_NOT_SUPPORTED.

Currently only defined for X86.

ZX_PROP_SOCKET_RX_THRESHOLD

handle type: Socket

value type: size_t

Allowed operations: get, set

The size of the read threshold of a socket, in bytes. Setting this will assert ZX_SOCKET_READ_THRESHOLD if the amount of data that can be read is greater than or equal to the threshold. Setting this property to zero will result in the deasserting of ZX_SOCKET_READ_THRESHOLD.

ZX_PROP_SOCKET_TX_THRESHOLD

handle type: Socket

value type: size_t

Allowed operations: get, set

The size of the write threshold of a socket, in bytes. Setting this will assert ZX_SOCKET_WRITE_THRESHOLD if the amount of space available for writing is greater than or equal to the threshold. Setting this property to zero will result in the deasserting of ZX_SOCKET_WRITE_THRESHOLD. Setting the write threshold after the peer has closed is an error, and results in a ZX_ERR_PEER_CLOSED error being returned.

ZX_PROP_JOB_KILL_ON_OOM

handle type: Job

value type: size_t

Allowed operations: set

The value of 1 means the Job and its children will be terminated if the system finds itself in a system-wide low memory situation. Called with 0 (which is the default) opts out the job from being terminated in this scenario.

ZX_PROP_EXCEPTION_STATE

handle type: Exception

value type: uint32_t

Allowed operations: get, set

When set to ZX_EXCEPTION_STATE_HANDLED, closing the exception handle will finish exception processing and resume the underlying thread. When set to ZX_EXCEPTION_STATE_TRY_NEXT, closing the exception handle will continue exception processing by trying the next handler in order. When set to ZX_EXCEPTION_STATE_THREAD_EXIT, closing the exception handle will cause the thread that generated the exception to exit.

ZX_PROP_EXCEPTION_STRATEGY

handle type: Exception

value type: uint32_t

Allowed operations: get, set

If ZX_EXCEPTION_STRATEGY_SECOND_CHANCE is set, then the debugger gets a 'second chance' at handling the exception if the process-level handler fails to do so.

This property can only be set when the handle corresponds to a debugger process exception channel. Attempting to set this property when the exception channel is any other type will result in ZX_ERR_BAD_STATE.

RIGHTS

handle must have ZX_RIGHT_GET_PROPERTY.

If property is ZX_PROP_PROCESS_DEBUG_ADDR, handle must be of type ZX_OBJ_TYPE_PROCESS.

If property is ZX_PROP_PROCESS_BREAK_ON_LOAD, handle must be of type ZX_OBJ_TYPE_PROCESS.

If property is ZX_PROP_PROCESS_VDSO_BASE_ADDRESS, handle must be of type ZX_OBJ_TYPE_PROCESS.

If property is ZX_PROP_SOCKET_RX_THRESHOLD, handle must be of type ZX_OBJ_TYPE_SOCKET.

If property is ZX_PROP_SOCKET_TX_THRESHOLD, handle must be of type ZX_OBJ_TYPE_SOCKET.

RETURN VALUE

zx_object_get_property() returns ZX_OK on success. In the event of failure, a negative error value is returned.

ERRORS

ZX_ERR_BAD_HANDLE: handle is not a valid handle

ZX_ERR_WRONG_TYPE: handle is not an appropriate type for property

ZX_ERR_ACCESS_DENIED: handle does not have the necessary rights for the operation

ZX_ERR_INVALID_ARGS: value is an invalid pointer

ZX_ERR_NO_MEMORY Failure due to lack of memory. There is no good way for userspace to handle this (unlikely) error. In a future build this error will no longer occur.

ZX_ERR_BUFFER_TOO_SMALL: value_size is too small for property

ZX_ERR_NOT_SUPPORTED: property does not exist

SEE ALSO