Hi Dave, On 6/24/20 7:36 PM, Dave Martin wrote: > Add documentation for the the PR_SET_TAGGED_ADDR_CTRL and > PR_GET_TAGGED_ADDR_CTRL prctls added in Linux 5.4 for arm64. > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx> > Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx> > Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> Thanks for another nicely written patch! I've applied locally, but won't push just yet, to allow for some (more) reviews/acks to come in. Thanks, Michael > --- > > Kept Catalin's Reviewed-by, since the changes are pretty minor. > > Changes since v2: > > * Clarified type of PR_SET_TAGGED_ADDR_CTRL arg2. > > * Added Linux commit where enforecement of zeroing reserved args was > added for PR_SET_TAGGED_ADDR_CTRL. > > * Added explicit text regarding how to disable the tagged-address ABI > globally through sysctl. > > * Rearrange the tagged argument syscall exclusion list so that shmdt() > isn't called out specially. The reader probably doesn't care about > this history here. > > * Minor rewordings. > --- > man2/prctl.2 | 161 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 161 insertions(+) > > diff --git a/man2/prctl.2 b/man2/prctl.2 > index 46ea9d2..cf92f3b 100644 > --- a/man2/prctl.2 > +++ b/man2/prctl.2 > @@ -1522,6 +1522,148 @@ For more information, see the kernel source file > (or > .I Documentation/arm64/sve.txt > before Linux 5.3). > +.\" prctl PR_SET_TAGGED_ADDR_CTRL > +.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d > +.TP > +.BR PR_SET_TAGGED_ADDR_CTRL " (since Linux 5.4, only on arm64)" > +Controls support for passing tagged userspace addresses to the kernel > +(i.e., addresses where bits 56\(em63 are not all zero). > +.IP > +The level of support is selected by > +.IR "arg2" , > +which can be one of the following: > +.RS > +.TP > +.B 0 > +Addresses that are passed > +for the purpose of being dereferenced by the kernel > +must be untagged. > +.TP > +.B PR_TAGGED_ADDR_ENABLE > +Addresses that are passed > +for the purpose of being dereferenced by the kernel > +may be tagged, with the exceptions summarized below. > +.RE > +.IP > +The remaining arguments > +.IR arg3 ", " arg4 " and " arg5 > +must all be zero. > +.\" Enforcement added in > +.\" commit 3e91ec89f527b9870fe42dcbdb74fd389d123a95 > +.IP > +On success, the mode specified in > +.I arg2 > +is set for the calling thread and the the return value is 0. > +If the arguments are invalid, > +the mode specified in > +.I arg2 > +is unrecognized, > +or if this feature is unsupported by the kernel > +or disabled via > +.IR /proc/sys/abi/tagged_addr_disabled , > +the call fails with > +.BR EINVAL . > +.IP > +In particular, if > +.BR prctl ( PR_SET_TAGGED_ADDR_CTRL , > +0, 0, 0, 0) > +fails with > +.B EINVAL > +then all addresses passed to the kernel must be untagged. > +.IP > +Irrespective of which mode is set, > +addresses passed to certain interfaces > +must always be untagged: > +.RS > +.IP \(em > +.BR brk (2), > +.BR mmap (2), > +.BR shmat (2), > +.BR shmdt (2), > +and the > +.I new_address > +argument of > +.BR mremap (2). > +.IP > +(Prior to Linux 5.6 these accepted tagged addresses, > +but the behaviour may not be what you expect. > +Don't rely on it.) > +.IP \(em > +\(oqpolymorphic\(cq interfaces > +that accept pointers to arbitrary types cast to a > +.I void * > +or other generic type, specifically > +.BR prctl (2), > +.BR ioctl (2), > +and in general > +.BR setsockopt (2) > +(only certain specific > +.BR setsockopt (2) > +options allow tagged addresses). > +.RE > +.IP > +This list of exclusions may shrink > +when moving from one kernel version to a later kernel version. > +While the kernel may make some guarantees > +for backwards compatibility reasons, > +for the purposes of new software > +the effect of passing tagged addresses to these interfaces > +is unspecified. > +.IP > +The mode set by this call is inherited across > +.BR fork (2) > +and > +.BR clone (2). > +The mode is reset by > +.BR execve (2) > +to 0 > +(i.e., tagged addresses not permitted in the user/kernel ABI). > +.IP > +For more information, see the kernel source file > +.IR Documentation/arm64/tagged\-address\-abi.rst . > +.IP > +.B Warning: > +This call is primarily intended for use by the run-time environment. > +A successful > +.B PR_SET_TAGGED_ADDR_CTRL > +call elsewhere may crash the calling process. > +The conditions for using it safely are complex and system-dependent; > +Don't use it unless you know what you are doing. > +.\" prctl PR_GET_TAGGED_ADDR_CTRL > +.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d > +.TP > +.BR PR_GET_TAGGED_ADDR_CTRL " (since Linux 5.4, only on arm64)" > +Returns the current tagged address mode > +for the calling thread. > +.IP > +Arguments > +.IR arg2 ", " arg3 ", " arg4 " and " arg5 > +must all be zero. > +.IP > +If the arguments are invalid > +or this feature is disabled or unsupported by the kernel, > +the call fails with > +.BR EINVAL . > +In particular, if > +.BR prctl ( PR_GET_TAGGED_ADDR_CTRL , > +0, 0, 0, 0) > +fails with > +.BR EINVAL , > +then this feature is definitely either unsupported, > +or disabled via > +.IR /proc/sys/abi/tagged_addr_disabled . > +In this case, > +all addresses passed to the kernel must be untagged. > +.IP > +Otherwise, the call returns a nonnegative value > +describing the current tagged address mode, > +encoded in the same way as the > +.I arg2 > +argument of > +.BR PR_SET_TAGGED_ADDR_CTRL . > +.IP > +For more information, see the kernel source file > +.IR Documentation/arm64/tagged\-address\-abi.rst . > .\" > .\" prctl PR_TASK_PERF_EVENTS_DISABLE > .TP > @@ -1767,6 +1909,7 @@ On success, > .BR PR_GET_SPECULATION_CTRL , > .BR PR_SVE_GET_VL , > .BR PR_SVE_SET_VL , > +.BR PR_GET_TAGGED_ADDR_CTRL , > .BR PR_GET_THP_DISABLE , > .BR PR_GET_TIMING , > .BR PR_GET_TIMERSLACK , > @@ -2074,6 +2217,24 @@ is > .B PR_SVE_GET_VL > and SVE is not available on this platform. > .TP > +.B EINVAL > +.I option > +is > +.BR PR_SET_TAGGED_ADDR_CTRL > +and the arguments are invalid or unsupported. > +See the description of > +.B PR_SET_TAGGED_ADDR_CTRL > +above for details. > +.TP > +.B EINVAL > +.I option > +is > +.BR PR_GET_TAGGED_ADDR_CTRL > +and the arguments are invalid or unsupported. > +See the description of > +.B PR_GET_TAGGED_ADDR_CTRL > +above for details. > +.TP > .B ENODEV > .I option > was > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
