Re: [musl] Incorrect thread TID caching

[Rich Felker <dalias@libc.org>]

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On Wed, Feb 17, 2021 at 03:11:57PM -0500, Rich Felker wrote:
> On Wed, Feb 17, 2021 at 02:49:45PM -0500, Dominic Chen wrote:
> > On 2/15/2021 11:56 AM, Rich Felker wrote:
> > >Following up on this now, the code in _Fork is something I really
> > >don't want to duplicate for clone() for risk of forgetting there's a
> > >copy in the latter and letting it bitrot there. I'd rather refactor
> > >things so the same logic can be shared...
> > 
> > Thanks for the update. Can you use something like
> > __attribute__((always_inline)) to just write the logic once but
> > force it to be inlined into both library functions?
> 
> Whether it's inlined isn't really a big deal; this is not a hot path.
> It's more just a matter of how it needs to be split up at the source
> level, and it seems to be messy whichever way we choose.
> 
> Trying to avoid calling __clone doesn't seem like such a good idea,
> since the child has to run on a new stack -- if we did avoid it we'd
> need a new way to switch stacks. The generic __unmapself has a hack
> to do this already that we could reuse without needing new
> arch-specific glue though.
> 
> I'll keep trying things and see if I come up with something not too
> unreasonable.

Attached is a draft of how clone() *could* work without refactoring
the pre/post logic from _Fork to use __clone. I don't particularly
like it, and CRTJMP abuse like this (which __unmapself also does, as
noted above) is not valid for FDPIC archs (it actually expects a code
address not a function pointer). I'll try a version the other way and
see how it looks.

Rich

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diff --git a/src/linux/clone.c b/src/linux/clone.c
index 8c1af7d3..32d53a8f 100644
--- a/src/linux/clone.c
+++ b/src/linux/clone.c
@@ -4,6 +4,25 @@
 #include <sched.h>
 #include "pthread_impl.h"
 #include "syscall.h"
+#include "dynlink.h"
+
+extern int _Forklike(int (*)(void *), void *);
+
+struct clone_args {
+	int flags;
+	pid_t *ptid, *ctid;
+};
+
+static int do_clone(void *p)
+{
+	struct clone_args *args = p;
+	int mask = CLONE_PARENT_SETTID | CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID;
+	int r = __syscall(SYS_clone, args->flags & ~mask, 0);
+	if (r>0 && (args->flags & CLONE_PARENT_SETTID)) *args->ptid = r;
+	if (!r && (args->flags & CLONE_CHILD_SETTID)) *args->ctid = __syscall(SYS_gettid);
+	if (!r && (args->flags & CLONE_CHILD_CLEARTID)) __syscall(SYS_set_tid_address, args->ctid);
+	return r;
+}
 
 int clone(int (*func)(void *), void *stack, int flags, void *arg, ...)
 {
@@ -17,5 +36,13 @@ int clone(int (*func)(void *), void *stack, int flags, void *arg, ...)
 	ctid = va_arg(ap, pid_t *);
 	va_end(ap);
 
+	if (!(flags & CLONE_VM)) {
+		struct clone_args args = { .flags = flags, .ptid = ptid, .ctid = ctid };
+		if (flags & CLONE_THREAD) return __syscall_ret(-EINVAL);
+		int r = _Forklike(do_clone, &args);
+		if (r) return r;
+		CRTJMP(func, stack);
+	}
+
 	return __syscall_ret(__clone(func, stack, flags, arg, ptid, tls, ctid));
 }