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([2001:a61:2467:2f01:faca:3d43:5e40:30d1]) by smtp.gmail.com with ESMTPSA id n11sm21225245wra.9.2020.12.24.01.18.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Dec 2020 01:18:56 -0800 (PST) Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org Subject: Re: [PATCH v5] prctl.2: Document Syscall User Dispatch To: Gabriel Krisman Bertazi , alx.manpages@gmail.com References: <20201223182911.4066380-1-krisman@collabora.com> From: "Michael Kerrisk (man-pages)" Message-ID: <85b23b60-a92c-a5a8-1cc5-24bdfb8b3530@gmail.com> Date: Thu, 24 Dec 2020 10:18:55 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20201223182911.4066380-1-krisman@collabora.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hello Gabriel, Thanks for the revisions! On 12/23/20 7:29 PM, Gabriel Krisman Bertazi wrote: > Signed-off-by: Gabriel Krisman Bertazi > > --- > 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/