mirror of https://github.com/mkerrisk/man-pages
prctl.2: Document Syscall User Dispatch
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Gabriel Krisman Bertazi
Michael Kerrisk
parent
8a6b6cb878
commit
131ee1e1de
154
man2/prctl.2
154
man2/prctl.2
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@ -1533,6 +1533,130 @@ For more information, see the kernel source file
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(or
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.I Documentation/arm64/sve.txt
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before Linux 5.3).
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.TP
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.\" prctl PR_SET_SYSCALL_USER_DISPATCH
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.\" commit 1446e1df9eb183fdf81c3f0715402f1d7595d4
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.BR PR_SET_SYSCALL_USER_DISPATCH " (since Linux 5.11, x86 only)"
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.IP
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Configure the Syscall User Dispatch mechanism
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for the calling thread.
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This mechanism allows an application
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to selectively intercept system calls
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so that they can be handled within the application itself.
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Interception takes the form of a thread-directed
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.B SIGSYS
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signal that is delivered to the thread
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when it makes a system call.
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If intercepted,
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the system call is not executed by the kernel.
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.IP
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To enable this mechanism,
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.I arg2
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should be set to
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.BR PR_SYS_DISPATCH_ON .
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Once enabled, further system calls will be selectively intercepted,
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depending on a control variable provided by user space.
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In this case,
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.I arg3
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and
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.I arg4
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respectively identify the
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.I offset
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and
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.I length
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of a single contiguous memory region in the process address space
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from where system calls are always allowed to be executed,
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regardless of the control variable.
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(Typically, this area would include the area of memory
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containing the C library.)
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.IP
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.I arg5
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points to a char-sized variable
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that is a fast switch to allow/block system call execution
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without the overhead of doing another system call
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to re-configure Syscall User Dispatch.
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This control variable can either be set to
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.B SYSCALL_DISPATCH_FILTER_BLOCK
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to block system calls from executing
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or to
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.B SYSCALL_DISPATCH_FILTER_ALLOW
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to temporarily allow them to be executed.
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This value is checked by the kernel
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on every system call entry,
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and any unexpected value will raise
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an uncatchable
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.B SIGSYS
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at that time,
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killing the application.
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.IP
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When a system call is intercepted,
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the kernel sends a thread-directed
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.B SIGSYS
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signal to the triggering thread.
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Various fields will be set in the
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.I siginfo_t
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structure (see
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.BR sigaction (2))
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associated with the signal:
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.RS
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.IP * 3
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.I si_signo
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will contain
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.BR SIGSYS .
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.IP *
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.IR si_call_addr
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will show the address of the system call instruction.
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.IP *
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.IR si_syscall
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and
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.IR si_arch
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will indicate which system call was attempted.
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.IP *
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.I si_code
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will contain
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.BR SYS_USER_DISPATCH .
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.IP *
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.I si_errno
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will be set to 0.
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.RE
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.IP
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The program counter will be as though the system call happened
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(i.e., the program counter will not point to the system call instruction).
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.IP
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When the signal handler returns to the kernel,
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the system call completes immediately
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and returns to the calling thread,
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without actually being executed.
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If necessary
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(i.e., when emulating the system call on user space.),
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the signal handler should set the system call return value
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to a sane value,
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by modifying the register context stored in the
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.I ucontext
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argument of the signal handler.
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See
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.BR sigaction (2),
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.BR sigreturn (2),
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and
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.BR getcontext (3)
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for more information.
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.IP
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If
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.I arg2
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is set to
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.BR PR_SYS_DISPATCH_OFF ,
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Syscall User Dispatch is disabled for that thread.
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the remaining arguments must be set to 0.
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.IP
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The setting is not preserved across
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.BR fork (2),
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.BR clone (2),
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or
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.BR execve (2).
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.IP
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For more information,
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see the kernel source file
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.IR Documentation/admin-guide/syscall-user-dispatch.rst
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.\" prctl PR_SET_TAGGED_ADDR_CTRL
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.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
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.TP
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@ -2000,6 +2124,14 @@ and
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.I arg3
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is an invalid address.
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.TP
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.B EFAULT
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.I option
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is
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.B PR_SET_SYSCALL_USER_DISPATCH
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and
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.I arg5
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has an invalid address.
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.TP
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.B EINVAL
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The value of
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.I option
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@ -2231,6 +2363,28 @@ and SVE is not available on this platform.
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.B EINVAL
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.I option
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is
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.B PR_SET_SYSCALL_USER_DISPATCH
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and one of the following is true:
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.RS
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.IP * 3
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.I arg2
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is
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.B PR_SYS_DISPATCH_OFF
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and the remaining arguments are not 0;
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.IP * 3
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.I arg2
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is
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.B PR_SYS_DISPATCH_ON
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and the memory range specified is outside the
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address space of the process.
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.IP * 3
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.I arg2
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is invalid.
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.RE
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.TP
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.B EINVAL
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.I option
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is
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.BR PR_SET_TAGGED_ADDR_CTRL
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and the arguments are invalid or unsupported.
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See the description of
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