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SubjectRe: [PATCH v8 3/7] kernel: Implement selective syscall userspace redirection
On Fri, Nov 27, 2020 at 02:32:34PM -0500, Gabriel Krisman Bertazi wrote:
> Introduce a mechanism to quickly disable/enable syscall handling for a
> specific process and redirect to userspace via SIGSYS. This is useful
> for processes with parts that require syscall redirection and parts that
> don't, but who need to perform this boundary crossing really fast,
> without paying the cost of a system call to reconfigure syscall handling
> on each boundary transition. This is particularly important for Windows
> games running over Wine.
> The proposed interface looks like this:
> prctl(PR_SET_SYSCALL_USER_DISPATCH, <op>, <off>, <length>, [selector])
> The range [<offset>,<offset>+<length>) is a part of the process memory
> map that is allowed to by-pass the redirection code and dispatch
> syscalls directly, such that in fast paths a process doesn't need to
> disable the trap nor the kernel has to check the selector. This is
> essential to return from SIGSYS to a blocked area without triggering
> another SIGSYS from rt_sigreturn.
> selector is an optional pointer to a char-sized userspace memory region
> that has a key switch for the mechanism. This key switch is set to
> either PR_SYS_DISPATCH_ON, PR_SYS_DISPATCH_OFF to enable and disable the
> redirection without calling the kernel.
> The feature is meant to be set per-thread and it is disabled on
> fork/clone/execv.
> Internally, this doesn't add overhead to the syscall hot path, and it
> requires very little per-architecture support. I avoided using seccomp,
> even though it duplicates some functionality, due to previous feedback
> that maybe it shouldn't mix with seccomp since it is not a security
> mechanism. And obviously, this should never be considered a security
> mechanism, since any part of the program can by-pass it by using the
> syscall dispatcher.
> For the sysinfo benchmark, which measures the overhead added to
> executing a native syscall that doesn't require interception, the
> overhead using only the direct dispatcher region to issue syscalls is
> pretty much irrelevant. The overhead of using the selector goes around
> 40ns for a native (unredirected) syscall in my system, and it is (as
> expected) dominated by the supervisor-mode user-address access. In
> fact, with SMAP off, the overhead is consistently less than 5ns on my
> test box.
> Cc: Matthew Wilcox <>
> Cc: Andy Lutomirski <>
> Cc: Paul Gofman <>
> Cc: Kees Cook <>
> Cc:
> Signed-off-by: Gabriel Krisman Bertazi <>

Acked-by: Kees Cook <>

Kees Cook

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