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freebsd-skq/share/man/man9/casuword.9

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Provide protection against starvation of the ll/sc loops when accessing userpace. Casueword(9) on ll/sc architectures must be prepared for userspace constantly modifying the same cache line as containing the CAS word, and not loop infinitely. Otherwise, rogue userspace livelocks the kernel. To fix the issue, change casueword(9) interface to return new value 1 indicating that either comparision or store failed, instead of relying on the oldval == *oldvalp comparison. The primitive no longer retries the operation if it failed spuriously. Modify callers of casueword(9), all in kern_umtx.c, to handle retries, and react to stops and requests to terminate between retries. On x86, despite cmpxchg should not return spurious failures, we can take advantage of the new interface and just return PSL.ZF. Reviewed by: andrew (arm64, previous version), markj Tested by: pho Reported by: https://xenbits.xen.org/xsa/advisory-295.txt Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Differential revision: https://reviews.freebsd.org/D20772
2019-07-12 18:43:24 +00:00
.\" Copyright (c) 2014, 2019 The FreeBSD Foundation
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.\" All rights reserved.
.\"
.\" Part of this documentation was written by
.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
.\" from the FreeBSD Foundation.
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Provide protection against starvation of the ll/sc loops when accessing userpace. Casueword(9) on ll/sc architectures must be prepared for userspace constantly modifying the same cache line as containing the CAS word, and not loop infinitely. Otherwise, rogue userspace livelocks the kernel. To fix the issue, change casueword(9) interface to return new value 1 indicating that either comparision or store failed, instead of relying on the oldval == *oldvalp comparison. The primitive no longer retries the operation if it failed spuriously. Modify callers of casueword(9), all in kern_umtx.c, to handle retries, and react to stops and requests to terminate between retries. On x86, despite cmpxchg should not return spurious failures, we can take advantage of the new interface and just return PSL.ZF. Reviewed by: andrew (arm64, previous version), markj Tested by: pho Reported by: https://xenbits.xen.org/xsa/advisory-295.txt Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Differential revision: https://reviews.freebsd.org/D20772
2019-07-12 18:43:24 +00:00
.Dd April 19, 2019
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.Dt CASU 9
.Os
.Sh NAME
.Nm casueword ,
.Nm casueword32 ,
.Nm casuword ,
.Nm casuword32
.Nd fetch, compare and store data from user-space
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.Ft int
Style: avoid long lines by using .Fo instead of .Fn. Sponsored by: The FreeBSD Foundation MFC after: 3 days
2019-07-12 18:39:41 +00:00
.Fo casueword
.Fa "volatile u_long *base"
.Fa "u_long oldval"
.Fa "u_long *oldvalp"
.Fa "u_long newval"
.Fc
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.Ft int
Style: avoid long lines by using .Fo instead of .Fn. Sponsored by: The FreeBSD Foundation MFC after: 3 days
2019-07-12 18:39:41 +00:00
.Fo casueword32
.Fa "volatile uint32_t *base"
.Fa "uint32_t oldval"
.Fa "uint32_t *oldvalp"
.Fa "uint32_t newval"
.Fc
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.Ft u_long
Style: avoid long lines by using .Fo instead of .Fn. Sponsored by: The FreeBSD Foundation MFC after: 3 days
2019-07-12 18:39:41 +00:00
.Fo casuword
.Fa "volatile u_long *base"
.Fa "u_long oldval"
.Fa "u_long newval"
.Fc
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.Ft uint32_t
Style: avoid long lines by using .Fo instead of .Fn. Sponsored by: The FreeBSD Foundation MFC after: 3 days
2019-07-12 18:39:41 +00:00
.Fo casuword32
.Fa "volatile uint32_t *base"
.Fa "uint32_t oldval"
.Fa "uint32_t newval"
.Fc
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.Sh DESCRIPTION
The
.Nm
functions are designed to perform atomic compare-and-swap operation on
the value in the usermode memory of the current process.
.Pp
The
.Nm
routines reads the value from user memory with address
.Pa base ,
and compare the value read with
.Pa oldval .
If the values are equal,
.Pa newval
is written to the
.Pa *base .
In case of
.Fn casueword32
and
.Fn casueword ,
old value is stored into the (kernel-mode) variable pointed by
.Pa *oldvalp .
The userspace value must be naturally aligned.
.Pp
The callers of
.Fn casuword
and
.Fn casuword32
functions cannot distinguish between -1 read from
userspace and function failure.
.Sh RETURN VALUES
The
.Fn casuword
and
.Fn casuword32
functions return the data fetched or -1 on failure.
The
.Fn casueword
and
.Fn casueword32
Provide protection against starvation of the ll/sc loops when accessing userpace. Casueword(9) on ll/sc architectures must be prepared for userspace constantly modifying the same cache line as containing the CAS word, and not loop infinitely. Otherwise, rogue userspace livelocks the kernel. To fix the issue, change casueword(9) interface to return new value 1 indicating that either comparision or store failed, instead of relying on the oldval == *oldvalp comparison. The primitive no longer retries the operation if it failed spuriously. Modify callers of casueword(9), all in kern_umtx.c, to handle retries, and react to stops and requests to terminate between retries. On x86, despite cmpxchg should not return spurious failures, we can take advantage of the new interface and just return PSL.ZF. Reviewed by: andrew (arm64, previous version), markj Tested by: pho Reported by: https://xenbits.xen.org/xsa/advisory-295.txt Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Differential revision: https://reviews.freebsd.org/D20772
2019-07-12 18:43:24 +00:00
functions return 0 on success, -1 on failure to access memory,
and 1 when comparison or store failed.
The store can fail on load-linked/store-conditional architectures.
Add fueword(9) and casueword(9) functions. They are like fuword(9) and casuword(9), but do not mix value read and indication of fault. I know (or remember) enough assembly to handle x86 and powerpc. For arm, mips and sparc64, implement fueword() and casueword() as wrappers around fuword() and casuword(), which means that the functions cannot distinguish between -1 and fault. On architectures where fueword() and casueword() are native, implement fuword() and casuword() using fueword() and casuword(), to reduce assembly code duplication. Sponsored by: The FreeBSD Foundation Tested by: pho MFC after: 2 weeks (ia64 needs treating)
2014-10-28 15:22:13 +00:00
.Sh SEE ALSO
.Xr atomic 9 ,
.Xr fetch 9 ,
.Xr store 9
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