Implement `fcntl` options `F_GETSIG` and `F_SETSIG`.

These options allow overriding the signal that gets sent to the process when
I/O operations are available on the file descriptor, rather than the default
`SIGIO` signal. Doing so also populates `siginfo` to contain extra information
about which file descriptor caused the event (`si_fd`) and what events happened
on it (`si_band`). The logic around which FD is populated within `si_fd`
matches Linux's, which means it has some weird edge cases where that value may
not actually refer to a file descriptor that is still valid.

This CL also ports extra S/R logic regarding async handler in VFS2.
Without this, async I/O handlers aren't properly re-registered after S/R.

PiperOrigin-RevId: 345436598

1 files changed, 23 insertions, 0 deletions

diff --git a/pkg/sentry/arch/signal.go b/pkg/sentry/arch/signal.go
index c9fb55d00..5138f3bf5 100644
--- a/pkg/sentry/arch/signal.go
+++ b/pkg/sentry/arch/signal.go
@@ -251,3 +251,26 @@ func (s *SignalInfo) Arch() uint32 {
 func (s *SignalInfo) SetArch(val uint32) {
 	usermem.ByteOrder.PutUint32(s.Fields[12:16], val)
 }
+
+// Band returns the si_band field.
+func (s *SignalInfo) Band() int64 {
+	return int64(usermem.ByteOrder.Uint64(s.Fields[0:8]))
+}
+
+// SetBand mutates the si_band field.
+func (s *SignalInfo) SetBand(val int64) {
+	// Note: this assumes the platform uses `long` as `__ARCH_SI_BAND_T`.
+	// On some platforms, which gVisor doesn't support, `__ARCH_SI_BAND_T` is
+	// `int`. See siginfo.h.
+	usermem.ByteOrder.PutUint64(s.Fields[0:8], uint64(val))
+}
+
+// FD returns the si_fd field.
+func (s *SignalInfo) FD() uint32 {
+	return usermem.ByteOrder.Uint32(s.Fields[8:12])
+}
+
+// SetFD mutates the si_fd field.
+func (s *SignalInfo) SetFD(val uint32) {
+	usermem.ByteOrder.PutUint32(s.Fields[8:12], val)
+}