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clone.2: Adjust syscall prototype and expand CLONE_SETTLS description 

The prototype for the system call was added in kernel commit
81f10dad, but looking at the kernel's fork.c, I believe the
relevant definition is

SYSCALL_DEFINE5(clone, unsigned long, clone_flags,
                unsigned long, newsp,
                int __user *, parent_tidptr,
                int __user *, child_tidptr,
                unsigned long, tls)

so the last argument is the tls argument, not a pt_regs argument.
I stumbled upon this while trying to understand CLONE_SETTLS, so
I expanded that description a little to cover other architectures.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Keno Fischer 2016-08-23 23:06:07 -04:00 committed by Michael Kerrisk
parent 2293a55f93
commit dd6d3d2e5f
1 changed files with 20 additions and 6 deletions
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26
man2/clone.2
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@ -51,14 +51,14 @@ clone, __clone2 \- create a child process
.BI "int clone(int (*" "fn" ")(void *), void *" child_stack ,
.BI " int " flags ", void *" "arg" ", ... "
.BI " /* pid_t *" ptid ", struct user_desc *" tls \
.BI " /* pid_t *" ptid ", void *" newtls \
", pid_t *" ctid " */ );"
/* Prototype for the raw system call */
.BI "long clone(unsigned long " flags ", void *" child_stack ,
.BI " void *" ptid ", void *" ctid ,
.BI " struct pt_regs *" regs );
.BI " unsigned long " newtls );
.fi
.SH DESCRIPTION
.BR clone ()
@ -544,11 +544,25 @@ then trace the child also (see
.BR ptrace (2)).
.TP
.BR CLONE_SETTLS " (since Linux 2.5.32)"
The
The TLS (Thread Local Storage) descriptor is set to
.I newtls.
The interpretation of
.I newtls
argument is the new TLS (Thread Local Storage) descriptor.
and the resulting effect is architecture dependent.
On x86,
.I newtls
is interpreted as a
.IR "struct user_desc *"
(See
.BR set_thread_area (2).)
.BR set_thread_area (2)).
On x86_64 it is the new value to be set for the %fs base register
(See the
.I ARCH_SET_FS
argument to
.BR arch_prctl (2)).
On architectures with a dedicated TLS register, it is the new value
of that register.
.TP
.BR CLONE_SIGHAND " (since Linux 2.0)"
If
@ -813,7 +827,7 @@ The raw system call interface on x86 and many other architectures is roughly:
.BI "long clone(unsigned long " flags ", void *" child_stack ,
.BI " void *" ptid ", void *" ctid ,
.BI " struct pt_regs *" regs );
.BI " unsigned long " newtls );
.fi
.in