better cwd handling of traced process

backend.h

@@ -34,16 +34,19 @@
 #include <stddef.h>
 #include <sys/types.h>
 
-/* Convert a PID to a path to the corresponding binary.  */
+/* Convert a pid to a path to the corresponding binary.  */
 char *pid2name(pid_t pid);
 
-/* Given a PID, find a leader of thread group.  */
+/* return the cwd of a pid.  */
+char *pid2cwd(pid_t pid);
+
+/* Given a pid, find a leader of thread group.  */
 pid_t process_leader(pid_t pid);
 
-/* Given a PID of leader thread, fill in PIDs of all the tasks.  The
- * function will initialize the pointer *RET_TASKS to a
+/* Given a pid of leader thread, fill in pids of all the tasks.  The
+ * function will initialize the pointer *ret_tasks to a
  * newly-allocated array, and will store number of elements in that
- * array to *RET_N.  You have to free that buffer when you don't need
+ * array to *ret_n.  You have to free that buffer when you don't need
  * it anymore.  */
 int process_tasks(pid_t pid, pid_t **ret_tasks, size_t *ret_n);
 
@@ -56,8 +59,7 @@ void trace_me(void);
 /* stop tracing a task.  */
 int untrace_task(struct task *task, int signum);
 
-/* Called when mtrace needs to attach to task, such as when it attaches
- * to a running process, whose PID is given on the command line.  */
+/* Called when mtrace needs to attach to task */
 int trace_attach(struct task *task);
 
 /* wait for a task ready for tracing */
@@ -127,7 +129,7 @@ int linkmap_init(struct task *task, arch_addr_t dyn_addr);
 
 /* This should extract entry point address and interpreter (dynamic
  * linker) bias if possible.  Returns 0 if there were no errors, -1
- * otherwise.  Sets *ENTRYP and *INTERP_BIASP to non-zero values if
+ * otherwise.  Sets *entryp and *interp_biasp to non-zero values if
  * the corresponding value is known, or zero otherwise; this is not
  * done for pointers that are NULL.  */
 int process_get_entry(struct task *task, unsigned long *entryp, unsigned long *interp_biasp);

mtelf.c

@@ -48,14 +48,32 @@
 #include "common.h"
 #include "report.h"
 
-static int open_elf(struct mt_elf *mte, const char *filename)
+static int open_elf(struct mt_elf *mte, struct task *task, const char *filename)
 {
-	mte->filename = filename;
+	char *cwd;
 
-	if (options.cwd != -1)
-		mte->fd = openat(options.cwd, filename, O_RDONLY);
-	else
-		mte->fd = open(filename, O_RDONLY);
+	mte->filename = filename;
+	mte->fd = -1;
+
+	cwd = pid2cwd(task->pid);
+	if (cwd) {
+		int fd = open(cwd, O_RDONLY|O_DIRECTORY);
+
+		if (fd != -1) {
+			mte->fd = openat(fd, filename, O_RDONLY);
+
+			close(fd);
+		}
+
+		free(cwd);
+	}
+
+	if (mte->fd == -1) {
+		if (options.cwd != -1)
+			mte->fd = openat(options.cwd, filename, O_RDONLY);
+		else
+			mte->fd = open(filename, O_RDONLY);
+	}
 
 	if (mte->fd == -1)
 		return 1;
@@ -236,11 +254,11 @@ static void close_elf(struct mt_elf *mte)
 	}
 }
 
-static int elf_read(struct mt_elf *mte, const char *filename, GElf_Addr bias)
+static int elf_read(struct mt_elf *mte, struct task *task, const char *filename, GElf_Addr bias)
 {
 	debug(DEBUG_FUNCTION, "filename=%s", filename);
 
-	if (open_elf(mte, filename) < 0)
+	if (open_elf(mte, task, filename) < 0)
 		return -1;
 
 	GElf_Phdr phdr;
@@ -351,7 +369,7 @@ int elf_read_library(struct task *task, struct library *lib, const char *filenam
 
 	library_set_filename(lib, filename);
 
-	if (elf_read(&mte, filename, bias) == -1)
+	if (elf_read(&mte, task, filename, bias) == -1)
 		return -1;
 
 	mte.bias = bias;
@@ -456,7 +474,7 @@ struct library *elf_read_main_binary(struct task *task)
 
 	library_set_filename(lib, strdup(fname));
 
-	if (elf_read(&mte, filename, 0) == -1)
+	if (elf_read(&mte, task, filename, 0) == -1)
 		goto fail3;
 
 	task->is_64bit = is_64bit(&mte);
@@ -490,7 +508,7 @@ struct library *elf_read_main_binary(struct task *task)
 
 	copy_str_from_proc(task, ARCH_ADDR_T(mte.interp), fname, sizeof(fname));
 
-	if (!elf_read(&mte_ld, fname, (GElf_Addr)base)) {
+	if (!elf_read(&mte_ld, task, fname, (GElf_Addr)base)) {
 		mte_ld.bias = (GElf_Addr)base;
 		mte_ld.entry_addr = mte_ld.ehdr.e_entry + (GElf_Addr)base;
 

sysdeps/linux-gnu/proc.c

@@ -71,6 +71,24 @@ char *pid2name(pid_t pid)
 	return strdup(proc_exe);
 }
 
+/*
+ * Returns a (malloc'd) file name corresponding to a running pid
+ */
+char *pid2cwd(pid_t pid)
+{
+	int ret;
+	char fname[PATH_MAX];
+	PROC_PID_FILE(proc_cwd, "/proc/%d/cwd", pid);
+
+	ret = readlink(proc_cwd, fname, sizeof(fname) - 1);
+	if (ret == -1)
+		return NULL;
+
+	fname[ret] = 0;
+
+	return strdup(fname);
+}
+
 static FILE *open_status_file(pid_t pid)
 {
 	PROC_PID_FILE(fn, "/proc/%d/status", pid);