4.4-stable review patch. If anyone has any objections, please let me know.
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From: Vineet Gupta vgupta@synopsys.com
commit c58a584f05e35d1d4342923cd7aac07d9c3d3d16 upstream.
Per ARC TLS ABI, r25 is designated TP (thread pointer register). However so far kernel didn't do any special treatment, like setting up usermode r25, even for CLONE_SETTLS. We instead relied on libc runtime to do this, in say clone libc wrapper [1]. This was deliberate to keep kernel ABI agnostic (userspace could potentially change TP, specially for different ARC ISA say ARCompact vs. ARCv2 with different spare registers etc)
However userspace setting up r25, after clone syscall opens a race, if child is not scheduled and gets a signal instead. It starts off in userspace not in clone but in a signal handler and anything TP sepcific there such as pthread_self() fails which showed up with uClibc testsuite nptl/tst-kill6 [2]
Fix this by having kernel populate r25 to TP value. So this locks in ABI, but it was not going to change anyways, and fwiw is same for both ARCompact (arc700 core) and ARCvs (HS3x cores)
[1] https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/libc/sysdeps/linux/ar... [2] https://github.com/wbx-github/uclibc-ng-test/blob/master/test/nptl/tst-kill6...
Fixes: ARC STAR 9001378481 Cc: stable@vger.kernel.org Reported-by: Nikita Sobolev sobolev@synopsys.com Signed-off-by: Vineet Gupta vgupta@synopsys.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/arc/kernel/process.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)
--- a/arch/arc/kernel/process.c +++ b/arch/arc/kernel/process.c @@ -153,6 +153,26 @@ int copy_thread(unsigned long clone_flag task_thread_info(current)->thr_ptr; }
+ + /* + * setup usermode thread pointer #1: + * when child is picked by scheduler, __switch_to() uses @c_callee to + * populate usermode callee regs: this works (despite being in a kernel + * function) since special return path for child @ret_from_fork() + * ensures those regs are not clobbered all the way to RTIE to usermode + */ + c_callee->r25 = task_thread_info(p)->thr_ptr; + +#ifdef CONFIG_ARC_CURR_IN_REG + /* + * setup usermode thread pointer #2: + * however for this special use of r25 in kernel, __switch_to() sets + * r25 for kernel needs and only in the final return path is usermode + * r25 setup, from pt_regs->user_r25. So set that up as well + */ + c_regs->user_r25 = c_callee->r25; +#endif + return 0; }