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libdm: Fix race conditions in LoopControl::Attach. 

This fixes two race conditions in LoopControl::Attach(). The first is
that after LOOP_CTL_GET_FREE, the path is not be available until it has
been processed by ueventd. This can be fixed by adding a timeout
parameter and a call to WaitForFile().

Second, it is possible (albeit unlikely), given that loop devices are
now being used more aggressively, that two processes race when
attempting LOOP_SET_FD. In this case, one process will win, and the
other will fail with EBUSY. We can handle this case by retrying the
operation while respecting the same timeout parameter.

Bug: 135771280
Test: libdm_test gtest
Change-Id: Icf9facc3ca28fdb6ff5c78612d3dc183fa47b1f3
This commit is contained in:
David Anderson 2019-07-11 13:09:42 -07:00
parent 470fe2b5f0
commit 4f9d1b15b4
8 changed files with 152 additions and 54 deletions
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1
fs_mgr/libdm/Android.bp
View File

@ -27,6 +27,7 @@ cc_library_static {
"dm_target.cpp",
"dm.cpp",
"loop_control.cpp",
"utility.cpp",
],
static_libs: [

27
fs_mgr/libdm/dm.cpp
View File

@ -29,6 +29,8 @@
#include <android-base/strings.h>
#include <uuid/uuid.h>
#include "utility.h"
namespace android {
namespace dm {
@ -94,20 +96,6 @@ bool DeviceMapper::DeleteDevice(const std::string& name) {
return true;
}
bool WaitForCondition(const std::function<bool()>& condition,
const std::chrono::milliseconds& timeout_ms) {
auto start_time = std::chrono::steady_clock::now();
while (true) {
if (condition()) return true;
std::this_thread::sleep_for(20ms);
auto now = std::chrono::steady_clock::now();
auto time_elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - start_time);
if (time_elapsed > timeout_ms) return false;
}
}
static std::string GenerateUuid() {
uuid_t uuid_bytes;
uuid_generate(uuid_bytes);
@ -138,16 +126,7 @@ bool DeviceMapper::CreateDevice(const std::string& name, const DmTable& table, s
if (timeout_ms <= std::chrono::milliseconds::zero()) {
return true;
}
auto condition = [&]() -> bool {
// If the file exists but returns EPERM or something, we consider the
// condition met.
if (access(unique_path.c_str(), F_OK) != 0) {
if (errno == ENOENT) return false;
}
return true;
};
if (!WaitForCondition(condition, timeout_ms)) {
if (!WaitForFile(unique_path, timeout_ms)) {
LOG(ERROR) << "Timed out waiting for device path: " << unique_path;
DeleteDevice(name);
return false;

8
fs_mgr/libdm/dm_test.cpp
View File

@ -103,9 +103,9 @@ TEST(libdm, DmLinear) {
ASSERT_TRUE(android::base::WriteFully(tmp1, message1, sizeof(message1)));
ASSERT_TRUE(android::base::WriteFully(tmp2, message2, sizeof(message2)));
LoopDevice loop_a(tmp1);
LoopDevice loop_a(tmp1, 10s);
ASSERT_TRUE(loop_a.valid());
LoopDevice loop_b(tmp2);
LoopDevice loop_b(tmp2, 10s);
ASSERT_TRUE(loop_b.valid());
// Define a 2-sector device, with each sector mapping to the first sector
@ -255,9 +255,9 @@ void SnapshotTestHarness::SetupImpl() {
cow_fd_ = CreateTempFile("cow_device", kCowDeviceSize);
ASSERT_GE(cow_fd_, 0);
base_loop_ = std::make_unique<LoopDevice>(base_fd_);
base_loop_ = std::make_unique<LoopDevice>(base_fd_, 10s);
ASSERT_TRUE(base_loop_->valid());
cow_loop_ = std::make_unique<LoopDevice>(cow_fd_);
cow_loop_ = std::make_unique<LoopDevice>(cow_fd_, 10s);
ASSERT_TRUE(cow_loop_->valid());
DmTable origin_table;

21
fs_mgr/libdm/include/libdm/loop_control.h
View File

@ -17,6 +17,7 @@
#ifndef _LIBDM_LOOP_CONTROL_H_
#define _LIBDM_LOOP_CONTROL_H_
#include <chrono>
#include <string>
#include <android-base/unique_fd.h>
@ -29,8 +30,15 @@ class LoopControl final {
LoopControl();
// Attaches the file specified by 'file_fd' to the loop device specified
// by 'loopdev'
bool Attach(int file_fd, std::string* loopdev) const;
// by 'loopdev'. It is possible that in between allocating and attaching
// a loop device, another process attaches to the chosen loop device. If
// this happens, Attach() will retry for up to |timeout_ms|. The timeout
// should not be zero.
//
// The caller does not have to call WaitForFile(); it is implicitly called.
// The given |timeout_ms| covers both potential sources of timeout.
bool Attach(int file_fd, const std::chrono::milliseconds& timeout_ms,
std::string* loopdev) const;
// Detach the loop device given by 'loopdev' from the attached backing file.
bool Detach(const std::string& loopdev) const;
@ -56,13 +64,13 @@ class LoopDevice {
public:
// Create a loop device for the given file descriptor. It is closed when
// LoopDevice is destroyed only if auto_close is true.
LoopDevice(int fd, bool auto_close = false);
LoopDevice(int fd, const std::chrono::milliseconds& timeout_ms, bool auto_close = false);
// Create a loop device for the given file path. It will be opened for
// reading and writing and closed when the loop device is detached.
explicit LoopDevice(const std::string& path);
LoopDevice(const std::string& path, const std::chrono::milliseconds& timeout_ms);
~LoopDevice();
bool valid() const { return fd_ != -1 && !device_.empty(); }
bool valid() const { return valid_; }
const std::string& device() const { return device_; }
LoopDevice(const LoopDevice&) = delete;
@ -71,12 +79,13 @@ class LoopDevice {
LoopDevice(LoopDevice&&) = default;
private:
void Init();
void Init(const std::chrono::milliseconds& timeout_ms);
android::base::unique_fd fd_;
bool owns_fd_;
std::string device_;
LoopControl control_;
bool valid_ = false;
};
} // namespace dm

61
fs_mgr/libdm/loop_control.cpp
View File

@ -27,6 +27,8 @@
#include <android-base/stringprintf.h>
#include <android-base/unique_fd.h>
#include "utility.h"
namespace android {
namespace dm {
@ -37,21 +39,40 @@ LoopControl::LoopControl() : control_fd_(-1) {
}
}
bool LoopControl::Attach(int file_fd, std::string* loopdev) const {
if (!FindFreeLoopDevice(loopdev)) {
LOG(ERROR) << "Failed to attach, no free loop devices";
return false;
}
bool LoopControl::Attach(int file_fd, const std::chrono::milliseconds& timeout_ms,
std::string* loopdev) const {
auto start_time = std::chrono::steady_clock::now();
auto condition = [&]() -> WaitResult {
if (!FindFreeLoopDevice(loopdev)) {
LOG(ERROR) << "Failed to attach, no free loop devices";
return WaitResult::Fail;
}
android::base::unique_fd loop_fd(TEMP_FAILURE_RETRY(open(loopdev->c_str(), O_RDWR | O_CLOEXEC)));
if (loop_fd < 0) {
PLOG(ERROR) << "Failed to open: " << *loopdev;
return false;
}
auto now = std::chrono::steady_clock::now();
auto time_elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - start_time);
if (!WaitForFile(*loopdev, timeout_ms - time_elapsed)) {
LOG(ERROR) << "Timed out waiting for path: " << *loopdev;
return WaitResult::Fail;
}
int rc = ioctl(loop_fd, LOOP_SET_FD, file_fd);
if (rc < 0) {
PLOG(ERROR) << "Failed LOOP_SET_FD";
android::base::unique_fd loop_fd(
TEMP_FAILURE_RETRY(open(loopdev->c_str(), O_RDWR | O_CLOEXEC)));
if (loop_fd < 0) {
PLOG(ERROR) << "Failed to open: " << *loopdev;
return WaitResult::Fail;
}
if (int rc = ioctl(loop_fd, LOOP_SET_FD, file_fd); rc == 0) {
return WaitResult::Done;
}
if (errno != EBUSY) {
PLOG(ERROR) << "Failed LOOP_SET_FD";
return WaitResult::Fail;
}
return WaitResult::Wait;
};
if (!WaitForCondition(condition, timeout_ms)) {
LOG(ERROR) << "Timed out trying to acquire a loop device";
return false;
}
return true;
@ -112,17 +133,19 @@ bool LoopControl::EnableDirectIo(int fd) {
return true;
}
LoopDevice::LoopDevice(int fd, bool auto_close) : fd_(fd), owns_fd_(auto_close) {
Init();
LoopDevice::LoopDevice(int fd, const std::chrono::milliseconds& timeout_ms, bool auto_close)
: fd_(fd), owns_fd_(auto_close) {
Init(timeout_ms);
}
LoopDevice::LoopDevice(const std::string& path) : fd_(-1), owns_fd_(true) {
LoopDevice::LoopDevice(const std::string& path, const std::chrono::milliseconds& timeout_ms)
: fd_(-1), owns_fd_(true) {
fd_.reset(open(path.c_str(), O_RDWR | O_CLOEXEC));
if (fd_ < -1) {
PLOG(ERROR) << "open failed for " << path;
return;
}
Init();
Init(timeout_ms);
}
LoopDevice::~LoopDevice() {
@ -134,8 +157,8 @@ LoopDevice::~LoopDevice() {
}
}
void LoopDevice::Init() {
control_.Attach(fd_, &device_);
void LoopDevice::Init(const std::chrono::milliseconds& timeout_ms) {
valid_ = control_.Attach(fd_, timeout_ms, &device_);
}
} // namespace dm

2
fs_mgr/libdm/loop_control_test.cpp
View File

@ -53,7 +53,7 @@ TEST(libdm, LoopControl) {
unique_fd fd = TempFile();
ASSERT_GE(fd, 0);
LoopDevice loop(fd);
LoopDevice loop(fd, 10s);
ASSERT_TRUE(loop.valid());
char buffer[6];

56
fs_mgr/libdm/utility.cpp Normal file
View File

@ -0,0 +1,56 @@
// Copyright (C) 2019 The Android Open Source Project
//
// 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.
#include "utility.h"
#include <errno.h>
#include <unistd.h>
#include <thread>
using namespace std::literals;
namespace android {
namespace dm {
bool WaitForCondition(const std::function<WaitResult()>& condition,
const std::chrono::milliseconds& timeout_ms) {
auto start_time = std::chrono::steady_clock::now();
while (true) {
auto result = condition();
if (result == WaitResult::Done) return true;
if (result == WaitResult::Fail) return false;
std::this_thread::sleep_for(20ms);
auto now = std::chrono::steady_clock::now();
auto time_elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(now - start_time);
if (time_elapsed > timeout_ms) return false;
}
}
bool WaitForFile(const std::string& path, const std::chrono::milliseconds& timeout_ms) {
auto condition = [&]() -> WaitResult {
// If the file exists but returns EPERM or something, we consider the
// condition met.
if (access(path.c_str(), F_OK) != 0) {
if (errno == ENOENT) return WaitResult::Wait;
}
return WaitResult::Done;
};
return WaitForCondition(condition, timeout_ms);
}
} // namespace dm
} // namespace android

30
fs_mgr/libdm/utility.h Normal file
View File

@ -0,0 +1,30 @@
// Copyright (C) 2019 The Android Open Source Project
//
// 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.
#pragma once
#include <chrono>
#include <functional>
namespace android {
namespace dm {
enum class WaitResult { Wait, Done, Fail };
bool WaitForFile(const std::string& path, const std::chrono::milliseconds& timeout_ms);
bool WaitForCondition(const std::function<WaitResult()>& condition,
const std::chrono::milliseconds& timeout_ms);
} // namespace dm
} // namespace android