Fallback to /proc/PID/mem when process_vm_readv not in kernel

news/240.feature.rst

@@ -0,0 +1 @@
+Support kernels that don't have CONFIG_CROSS_MEMORY_ATTACH set.

src/pystack/_pystack/mem.cpp

@@ -17,7 +17,6 @@
 namespace pystack {
 
 using elf_unique_ptr = std::unique_ptr<Elf, std::function<void(Elf*)>>;
-using file_unique_ptr = std::unique_ptr<FILE, std::function<int(FILE*)>>;
 
 static ssize_t
 _process_vm_readv(
@@ -228,6 +227,16 @@ ProcessMemoryManager::ProcessMemoryManager(pid_t pid)
 
 ssize_t
 ProcessMemoryManager::readChunk(remote_addr_t addr, size_t len, char* dst) const
+{
+    if (d_memfile || getenv("_PYSTACK_NO_PROCESS_VM_READV") != nullptr) {
+        return readChunkThroughMemFile(addr, len, dst);
+    } else {
+        return readChunkDirect(addr, len, dst);
+    }
+}
+
+ssize_t
+ProcessMemoryManager::readChunkDirect(remote_addr_t addr, size_t len, char* dst) const
 {
     struct iovec local[1];
     struct iovec remote[1];
@@ -246,6 +255,9 @@ ProcessMemoryManager::readChunk(remote_addr_t addr, size_t len, char* dst) const
                 throw InvalidRemoteAddress();
             } else if (errno == EPERM) {
                 throw std::runtime_error(PERM_MESSAGE);
+            } else if (errno == ENOSYS) {
+                LOG(DEBUG) << "process_vm_readv not compiled in kernel, falling back to /proc/PID/mem";
+                return readChunkThroughMemFile(addr, len, dst);
             }
             throw std::system_error(errno, std::generic_category());
         }
@@ -256,6 +268,30 @@ ProcessMemoryManager::readChunk(remote_addr_t addr, size_t len, char* dst) const
     return result;
 }
 
+ssize_t
+ProcessMemoryManager::readChunkThroughMemFile(remote_addr_t addr, size_t len, char* dst) const
+{
+    if (!d_memfile) {
+        std::string filepath = "/proc/" + std::to_string(d_pid) + "/mem";
+        d_memfile = file_unique_ptr(fopen(filepath.c_str(), "r"), fclose);
+        if (!d_memfile) {
+            if (errno == EPERM || errno == EACCES) {
+                LOG(ERROR) << "Permission denied opening file " << filepath;
+                throw std::runtime_error(PERM_MESSAGE);
+            }
+            LOG(ERROR) << "Failed to open file " << filepath << ": " << std::strerror(errno);
+            throw std::runtime_error("Failed to open " + filepath);
+        }
+    }
+    fseeko(d_memfile.get(), addr, SEEK_SET);
+    if (static_cast<off_t>(addr) != ftello(d_memfile.get())
+        || len != fread(dst, 1, len, d_memfile.get()))
+    {
+        throw InvalidRemoteAddress();
+    }
+    return static_cast<ssize_t>(len);
+}
+
 ssize_t
 ProcessMemoryManager::copyMemoryFromProcess(remote_addr_t addr, size_t len, void* dst) const
 {

src/pystack/_pystack/mem.h

@@ -12,9 +12,11 @@
 #include <sys/stat.h>
 #include <vector>
 
-#include <elf_common.h>
+#include "elf_common.h"
 
 namespace pystack {
+
+using file_unique_ptr = std::unique_ptr<FILE, std::function<int(FILE*)>>;
 typedef uintptr_t remote_addr_t;
 
 struct RemoteMemCopyError : public std::exception
@@ -174,9 +176,12 @@ class ProcessMemoryManager : public AbstractRemoteMemoryManager
     pid_t d_pid;
     std::vector<VirtualMap> d_vmaps;
     mutable LRUCache d_lru_cache;
+    mutable file_unique_ptr d_memfile;
 
     // Methods
     ssize_t readChunk(remote_addr_t addr, size_t len, char* dst) const;
+    ssize_t readChunkDirect(remote_addr_t addr, size_t len, char* dst) const;
+    ssize_t readChunkThroughMemFile(remote_addr_t addr, size_t len, char* dst) const;
 };
 
 struct SimpleVirtualMap

tests/integration/test_smoke.py

@@ -38,13 +38,23 @@
 
 @pytest.mark.parametrize("method", STACK_METHODS)
 @pytest.mark.parametrize("blocking", [True, False], ids=["blocking", "non-blocking"])
-def test_simple_execution(method, blocking, tmpdir):
+@pytest.mark.parametrize(
+    "use_process_vm_readv", [True, False], ids=["process_vm_readv", "/proc/PID/mem"]
+)
+def test_simple_execution(method, blocking, use_process_vm_readv, tmpdir, monkeypatch):
     """Test that we can retrieve the thread state of a single process.
 
     This test is specially useful to run under valgrind or similar tools and to
     quickly check that the very basic functionality works as expected.
     """
 
+    # GIVEN
+    env_var = "_PYSTACK_NO_PROCESS_VM_READV"
+    if use_process_vm_readv:
+        monkeypatch.delenv(env_var, raising=False)
+    else:
+        monkeypatch.setenv(env_var, "1")
+
     # WHEN
     with spawn_child_process(
         sys.executable, TEST_SINGLE_THREAD_FILE, tmpdir