Use open instead of fopen to avoid calls to malloc

If we are livepatching malloc we must be sure that we don't call it
during the process of writing the prologue, hence use `open` and
file descriptors instead of glibc `fopen`.

Signed-off-by: Giuliano Belinassi <[email protected]>

include/ulp.h

@@ -81,7 +81,7 @@ int __ulp_apply_patch();
 void __ulp_print();
 
 /* functions */
-FILE *get_proc_mem(void);
+int get_proc_mem(void);
 
 void free_metadata(struct ulp_metadata *ulp);
 

lib/interpose.c

@@ -583,8 +583,8 @@ maybe_disable_livepatching_on_group(void)
 static void
 check_proc_mem_is_accessibe(void)
 {
-  FILE *f = get_proc_mem();
-  if (f == NULL) {
+  int fd = get_proc_mem();
+  if (fd < 0) {
     WARN("/proc/self/mem is inacessible: %s", libpulp_strerror(errno));
     set_libpulp_error_state(EPROCMEM);
   }

lib/ulp.c

@@ -88,30 +88,31 @@ begin(void)
   msgq_push("libpulp loaded...\n");
 }
 
-/** @brief Attempt to get FILE pointer to /proc/self/mem
+/** @brief Attempt to get file descriptor to /proc/self/mem
  *
  * Chroots may change the path to the /proc folder.  This function attempts to
  * discover it.
  *
  * @return file pointer.
  */
-FILE *
+int
 get_proc_mem(void)
 {
-  FILE *file = fopen("/proc/self/mem", "r+");
+  int fd = open("/proc/self/mem", O_WRONLY);
 
   /* SLE have some processes which chroots into /proc.  If the above fopen
      fails then try this to check if this is the case.  */
-  if (file == NULL && errno == ENOENT) {
+  if (fd < 0 && errno == ENOENT) {
     /* Process has chroot'ed.  Check if it was chroot'ed to /proc itself.  */
-    file = fopen("/self/mem", "r+");
+    fd = open("/self/mem", O_WRONLY);
   }
 
-  if (file == NULL) {
+  if (fd < 0) {
     WARN("Attempting to open /proc/self/mem failed: %s", libpulp_strerror(errno));
+    set_libpulp_error_state(EPROCMEM);
   }
 
-  return file;
+  return fd;
 }
 
 /** @brief Write into memory bypassing memory protections
@@ -128,14 +129,18 @@ get_proc_mem(void)
 void *
 memwrite(void *dest, const void *src, size_t n)
 {
-  FILE *file = get_proc_mem();
-  libpulp_assert(file);
+  int fd = get_proc_mem();
+  off_t dst = (off_t) dest;
 
-  libpulp_assert(fseek(file, (size_t)dest, SEEK_SET) == 0);
-  libpulp_assert(fwrite(src, 1, n, file) == n);
+  if (fd < 0) {
+    return NULL;
+  }
 
-  fflush(file);
-  fclose(file);
+  libpulp_assert(lseek(fd, dst, SEEK_SET) == dst);
+  libpulp_assert(write(fd, src, n) == (ssize_t) n);
+
+  fsync(fd);
+  close(fd);
 
   return dest;
 }