Format fd_set parameters in select(2)/pselect(2) for strace.

I1202 14:55:06.835076    7991 x:0] [   1] select_test E
  select(0xa, 0x7fc6ce924c28 [0 1], null, null, 0x7fc6ce924c08 {sec=0 usec=0})
I1202 14:55:06.835102    7991 x:0] [   1] select_test X
  select(0xa, 0x7fc6ce924c28 [0 1], null, null, 0x7fc6ce924c08 {sec=0 usec=0})

PiperOrigin-RevId: 284831805

6 files changed, 96 insertions, 39 deletions

diff --git a/pkg/sentry/strace/BUILD b/pkg/sentry/strace/BUILD
index 72ebf766d..d46421199 100644
--- a/pkg/sentry/strace/BUILD
+++ b/pkg/sentry/strace/BUILD
@@ -14,6 +14,7 @@ go_library(
         "open.go",
         "poll.go",
         "ptrace.go",
+        "select.go",
         "signal.go",
         "socket.go",
         "strace.go",
diff --git a/pkg/sentry/strace/linux64.go b/pkg/sentry/strace/linux64.go
index f2763b3f1..e603f858f 100644
--- a/pkg/sentry/strace/linux64.go
+++ b/pkg/sentry/strace/linux64.go
@@ -40,7 +40,7 @@ var linuxAMD64 = SyscallMap{
 	20:  makeSyscallInfo("writev", FD, WriteIOVec, Hex),
 	21:  makeSyscallInfo("access", Path, Oct),
 	22:  makeSyscallInfo("pipe", PipeFDs),
-	23:  makeSyscallInfo("select", Hex, Hex, Hex, Hex, Timeval),
+	23:  makeSyscallInfo("select", Hex, SelectFDSet, SelectFDSet, SelectFDSet, Timeval),
 	24:  makeSyscallInfo("sched_yield"),
 	25:  makeSyscallInfo("mremap", Hex, Hex, Hex, Hex, Hex),
 	26:  makeSyscallInfo("msync", Hex, Hex, Hex),
@@ -287,7 +287,7 @@ var linuxAMD64 = SyscallMap{
 	267: makeSyscallInfo("readlinkat", FD, Path, ReadBuffer, Hex),
 	268: makeSyscallInfo("fchmodat", FD, Path, Mode),
 	269: makeSyscallInfo("faccessat", FD, Path, Oct, Hex),
-	270: makeSyscallInfo("pselect6", Hex, Hex, Hex, Hex, Hex, Hex),
+	270: makeSyscallInfo("pselect6", Hex, SelectFDSet, SelectFDSet, SelectFDSet, Timespec, SigSet),
 	271: makeSyscallInfo("ppoll", PollFDs, Hex, Timespec, SigSet, Hex),
 	272: makeSyscallInfo("unshare", CloneFlags),
 	273: makeSyscallInfo("set_robust_list", Hex, Hex),
diff --git a/pkg/sentry/strace/select.go b/pkg/sentry/strace/select.go
new file mode 100644
index 000000000..92c18083d
--- /dev/null
+++ b/pkg/sentry/strace/select.go
@@ -0,0 +1,53 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package strace
+
+import (
+	"fmt"
+
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/syscalls/linux"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+)
+
+func fdsFromSet(t *kernel.Task, set []byte) []int {
+	var fds []int
+	// Append n if the n-th bit is 1.
+	for i, v := range set {
+		for j := 0; j < 8; j++ {
+			if (v>>uint(j))&1 == 1 {
+				fds = append(fds, i*8+j)
+			}
+		}
+	}
+	return fds
+}
+
+func fdSet(t *kernel.Task, nfds int, addr usermem.Addr) string {
+	if addr == 0 {
+		return "null"
+	}
+
+	// Calculate the size of the fd set (one bit per fd).
+	nBytes := (nfds + 7) / 8
+	nBitsInLastPartialByte := uint(nfds % 8)
+
+	set, err := linux.CopyInFDSet(t, addr, nBytes, nBitsInLastPartialByte)
+	if err != nil {
+		return fmt.Sprintf("%#x (error decoding fdset: %s)", addr, err)
+	}
+
+	return fmt.Sprintf("%#x %v", addr, fdsFromSet(t, set))
+}
diff --git a/pkg/sentry/strace/strace.go b/pkg/sentry/strace/strace.go
index 311389547..629c1f308 100644
--- a/pkg/sentry/strace/strace.go
+++ b/pkg/sentry/strace/strace.go
@@ -439,6 +439,8 @@ func (i *SyscallInfo) pre(t *kernel.Task, args arch.SyscallArguments, maximumBlo
 			output = append(output, capData(t, args[arg-1].Pointer(), args[arg].Pointer()))
 		case PollFDs:
 			output = append(output, pollFDs(t, args[arg].Pointer(), uint(args[arg+1].Uint()), false))
+		case SelectFDSet:
+			output = append(output, fdSet(t, int(args[0].Int()), args[arg].Pointer()))
 		case Oct:
 			output = append(output, "0o"+strconv.FormatUint(args[arg].Uint64(), 8))
 		case Hex:
diff --git a/pkg/sentry/strace/syscalls.go b/pkg/sentry/strace/syscalls.go
index 3c389d375..e5d486c4e 100644
--- a/pkg/sentry/strace/syscalls.go
+++ b/pkg/sentry/strace/syscalls.go
@@ -206,6 +206,10 @@ const (
 	// PollFDs is an array of struct pollfd. The number of entries in the
 	// array is in the next argument.
 	PollFDs
+
+	// SelectFDSet is an fd_set argument in select(2)/pselect(2). The number of
+	// fds represented must be the first argument.
+	SelectFDSet
 )
 
 // defaultFormat is the syscall argument format to use if the actual format is
diff --git a/pkg/sentry/syscalls/linux/sys_poll.go b/pkg/sentry/syscalls/linux/sys_poll.go
index 7a13beac2..631dffec6 100644
--- a/pkg/sentry/syscalls/linux/sys_poll.go
+++ b/pkg/sentry/syscalls/linux/sys_poll.go
@@ -197,53 +197,51 @@ func doPoll(t *kernel.Task, addr usermem.Addr, nfds uint, timeout time.Duration)
 	return remainingTimeout, n, err
 }
 
+// CopyInFDSet copies an fd set from select(2)/pselect(2).
+func CopyInFDSet(t *kernel.Task, addr usermem.Addr, nBytes int, nBitsInLastPartialByte uint) ([]byte, error) {
+	set := make([]byte, nBytes)
+
+	if addr != 0 {
+		if _, err := t.CopyIn(addr, &set); err != nil {
+			return nil, err
+		}
+		// If we only use part of the last byte, mask out the extraneous bits.
+		//
+		// N.B. This only works on little-endian architectures.
+		if nBitsInLastPartialByte != 0 {
+			set[nBytes-1] &^= byte(0xff) << nBitsInLastPartialByte
+		}
+	}
+	return set, nil
+}
+
 func doSelect(t *kernel.Task, nfds int, readFDs, writeFDs, exceptFDs usermem.Addr, timeout time.Duration) (uintptr, error) {
 	if nfds < 0 || nfds > fileCap {
 		return 0, syserror.EINVAL
 	}
 
-	// Capture all the provided input vectors.
-	//
-	// N.B. This only works on little-endian architectures.
-	byteCount := (nfds + 7) / 8
-
-	bitsInLastPartialByte := uint(nfds % 8)
-	r := make([]byte, byteCount)
-	w := make([]byte, byteCount)
-	e := make([]byte, byteCount)
+	// Calculate the size of the fd sets (one bit per fd).
+	nBytes := (nfds + 7) / 8
+	nBitsInLastPartialByte := uint(nfds % 8)
 
-	if readFDs != 0 {
-		if _, err := t.CopyIn(readFDs, &r); err != nil {
-			return 0, err
-		}
-		// Mask out bits above nfds.
-		if bitsInLastPartialByte != 0 {
-			r[byteCount-1] &^= byte(0xff) << bitsInLastPartialByte
-		}
+	// Capture all the provided input vectors.
+	r, err := CopyInFDSet(t, readFDs, nBytes, nBitsInLastPartialByte)
+	if err != nil {
+		return 0, err
 	}
-
-	if writeFDs != 0 {
-		if _, err := t.CopyIn(writeFDs, &w); err != nil {
-			return 0, err
-		}
-		if bitsInLastPartialByte != 0 {
-			w[byteCount-1] &^= byte(0xff) << bitsInLastPartialByte
-		}
+	w, err := CopyInFDSet(t, writeFDs, nBytes, nBitsInLastPartialByte)
+	if err != nil {
+		return 0, err
 	}
-
-	if exceptFDs != 0 {
-		if _, err := t.CopyIn(exceptFDs, &e); err != nil {
-			return 0, err
-		}
-		if bitsInLastPartialByte != 0 {
-			e[byteCount-1] &^= byte(0xff) << bitsInLastPartialByte
-		}
+	e, err := CopyInFDSet(t, exceptFDs, nBytes, nBitsInLastPartialByte)
+	if err != nil {
+		return 0, err
 	}
 
 	// Count how many FDs are actually being requested so that we can build
 	// a PollFD array.
 	fdCount := 0
-	for i := 0; i < byteCount; i++ {
+	for i := 0; i < nBytes; i++ {
 		v := r[i] | w[i] | e[i]
 		for v != 0 {
 			v &= (v - 1)
@@ -254,7 +252,7 @@ func doSelect(t *kernel.Task, nfds int, readFDs, writeFDs, exceptFDs usermem.Add
 	// Build the PollFD array.
 	pfd := make([]linux.PollFD, 0, fdCount)
 	var fd int32
-	for i := 0; i < byteCount; i++ {
+	for i := 0; i < nBytes; i++ {
 		rV, wV, eV := r[i], w[i], e[i]
 		v := rV | wV | eV
 		m := byte(1)
@@ -295,8 +293,7 @@ func doSelect(t *kernel.Task, nfds int, readFDs, writeFDs, exceptFDs usermem.Add
 	}
 
 	// Do the syscall, then count the number of bits set.
-	_, _, err := pollBlock(t, pfd, timeout)
-	if err != nil {
+	if _, _, err = pollBlock(t, pfd, timeout); err != nil {
 		return 0, syserror.ConvertIntr(err, syserror.EINTR)
 	}