.

wcfs/wcfs.go

@@ -323,7 +323,7 @@ package main
 //
 //	remmapping is done synchronously via ptrace.
 //	XXX via running wcfs-trusted code wcfs injects into clients.
-//	FIXME ptrace won't work when client thread is blocked under pagefault or syscall.
+//	FIXME ptrace won't work when client thread is blocked under pagefault or syscall(+).
 //
 //	in order to support remmapping for each file/head/data
 //
@@ -403,6 +403,26 @@ package main
 //	but before that the analysis remains FUSE-specific.
 //
 //
+// (+) the kernel sends SIGSTOP to interrupt tracee, but the signal will be
+// processed only when the process returns from kernel space, e.g. here
+//
+//	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/entry/common.c?id=v4.19-rc8-151-g23469de647c4#n160
+//
+// This way the tracer won't receive obligatory information that tracee
+// stopped (via wait...) and even though ptrace(ATTACH) succeeds, all other
+// ptrace commands will fail:
+//
+//	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/ptrace.c?id=v4.19-rc8-151-g23469de647c4#n1140
+//	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/ptrace.c?id=v4.19-rc8-151-g23469de647c4#n207
+//
+// My original idea was to use ptrace to run code in process to change it's
+// memory mappings, while the triggering process is under pagefault/read
+// to wcfs, and the above shows it won't work - trying to ptrace the
+// client from under wcfs will just block forever (the kernel will be
+// waiting for read operation to finish for ptrace, and read will be first
+// waiting on ptrace stopping to complete = deadlock)
+//
+//
 // XXX mmap(@at) open
 //
 // XXX zconn(s) for historical state