Hello Gabriel,
Thanks for the revisions!
On 12/23/20 7:29 PM, Gabriel Krisman Bertazi wrote:
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
>
> ---
> Changes since v4:
> (suggested by Michael Kerrisk)
> - Modify explanation of what dispatch to user space means.
> - Drop references to emulation.
> - Document suggestion about placing libc in allowed-region.
> - Comment about avoiding syscall cost.
> Changes since v3:
> (suggested by Michael Kerrisk)
> - Explain what dispatch to user space means.
> - Document the fact that the memory region is a single consecutive
> range.
> - Explain failure if *arg5 is set to a bad value.
> - fix english typo.
> - Define what 'invalid memory region' means.
>
> Changes since v2:
> (suggested by Alejandro Colomar)
> - selective -> selectively
> - Add missing oxford comma.
>
> Changes since v1:
> (suggested by Alejandro Colomar)
> - Use semantic lines
> - Fix usage of .{B|I}R and .{B|I}
> - Don't format literals
> - Fix preferred spelling of userspace
> - Fix case of word
> ---
> man2/prctl.2 | 116 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 116 insertions(+)
>
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index f25f05fdb593..0d26fdcb5e3d 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -1533,6 +1533,92 @@ For more information, see the kernel source file
> (or
> .I Documentation/arm64/sve.txt
> before Linux 5.3).
> +.TP
> +.\" prctl PR_SET_SYSCALL_USER_DISPATCH
> +.\" commit 1446e1df9eb183fdf81c3f0715402f1d7595d4
> +.BR PR_SET_SYSCALL_USER_DISPATCH " (since Linux 5.11, x86 only)"
> +.IP
> +Configure the Syscall User Dispatch mechanism
> +for the calling thread.
> +This mechanism allows an application
> +to selectively intercept system calls
> +so that they can be handled within the application itself.
> +Interception takes the form of a thread-directed
> +.B SIGSYS
> +signal that is delivered to the thread
> +when it makes a system call.
> +If intercepted,
> +the system call is not executed by the kernel.
> +.IP
> +The current Syscall User Dispatch mode is selected via
> +.IR arg2 ,
> +which can either be set to
> +.B PR_SYS_DISPATCH_ON
> +to enable the feature,
> +or to
> +.B PR_SYS_DISPATCH_OFF
> +to turn it off.
> +.IP
> +When
> +.I arg2
> +is set to
> +.BR PR_SYS_DISPATCH_ON ,
> +.I arg3
> +and
> +.I arg4
> +respectively identify the
> +.I offset
> +and
> +.I length
> +of a single contiguous memory region in the process map
> +from where system calls are always allowed to be executed,
> +regardless of the switch variable
> +(Typically, this area would include the area of memory
> +containing the C library).
s/)./.)/
> +.I arg5
> +points to a char-sized variable
> +that is a fast switch to enable/disable the mechanism
> +without the overhead of doing a system call.
> +The variable pointed by
s/pointed/pointed to/
> +.I arg5
> +can either be set to
> +.B PR_SYS_DISPATCH_ON
> +to enable the mechanism
> +or to
> +.B PR_SYS_DISPATCH_OFF
> +to temporarily disable it.
> +The value pointed by
> +.B arg5
> +is checked by the kernel
> +on every system call entry,
> +and any unexpected value will raise
> +an uncatchable
> +.B SIGSYS
> +at that time,
> +killing the application.
> +.PI
> +When a system call is intercepted,
> +.B SIGSYS
> +is raised with
> +.I si_code
> +set to
> +.BR SYS_USER_DISPATCH .
Shouldn't we mention here also: si_call_addr, si_arch, and si_syscall?
So, by this point, there still seems to be info lacking.
When the SIGSYS handler returns, what happens? In the emulation
use-case, what will the return value of the system call look like?
Can that return value be defined from the SISGSYS handler, and
if so, how? I couldn't really see any answers in
Documentation/admin-guide/syscall-user-dispatch.rst
and I think there should be at least some of this info in the manual
page.
Cheers,
Michael
> +.IP
> +When
> +.I arg2
> +is set to
> +.BR PR_SYS_DISPATCH_OFF ,
> +the remaining arguments must be set to 0.
> +.IP
> +The setting is not preserved across
> +.BR fork (2),
> +.BR clone (2),
> +or
> +.BR execve (2).
> +.IP
> +For more information,
> +see the kernel source file
> +.IR Documentation/admin-guide/syscall-user-dispatch.rst
> .\" prctl PR_SET_TAGGED_ADDR_CTRL
> .\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
> .TP
> @@ -2000,6 +2086,14 @@ and
> .I arg3
> is an invalid address.
> .TP
> +.B EFAULT
> +.I option
> +is
> +.B PR_SET_SYSCALL_USER_DISPATCH
> +and
> +.I arg5
> +has an invalid address.
> +.TP
> .B EINVAL
> The value of
> .I option
> @@ -2231,6 +2325,28 @@ and SVE is not available on this platform.
> .B EINVAL
> .I option
> is
> +.B PR_SET_SYSCALL_USER_DISPATCH
> +and one of the following is true:
> +.RS
> +.IP * 3
> +.I arg2
> +is
> +.B PR_SYS_DISPATCH_OFF
> +and the remaining arguments are not 0;
> +.IP * 3
> +.I arg2
> +is
> +.B PR_SYS_DISPATCH_ON
> +and the memory range specified is outside the
> +address space of the process.
> +.IP * 3
> +.I arg2
> +is invalid.
> +.RE
> +.TP
> +.B EINVAL
> +.I option
> +is
> .BR PR_SET_TAGGED_ADDR_CTRL
> and the arguments are invalid or unsupported.
> See the description of
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
