efivars: fix concurrent growth read accounting

efi_get_variable() allocates one byte for probing whether efivarfs
has grown since fstat(), then three more bytes for NUL termination.
Account for both sizes separately so a full readv() result is treated
as concurrent growth and retried before the terminators are written.

Follow-up for ab69a04600

src/basic/efivars.c

@@ -87,14 +87,21 @@ int efi_get_variable(
                 }
 
                 /* We want +1 for the read call, and +3 for the additional terminating bytes added below. */
+                size_t file_size = (size_t) st.st_size, payload_size, read_size, alloc_size, read_limit;
+                if (!SUB_SAFE(&payload_size, file_size, sizeof(attr)) ||
+                    !ADD_SAFE(&read_size, payload_size, 1) ||
+                    !ADD_SAFE(&alloc_size, read_size, 3) ||
+                    !ADD_SAFE(&read_limit, file_size, 1))
+                        return log_debug_errno(SYNTHETIC_ERRNO(EOVERFLOW), "EFI variable '%s' size calculation overflow, refusing.", p);
+
                 free(buf);
-                buf = malloc((size_t) st.st_size - sizeof(attr) + CONST_MAX(1, 3));
+                buf = malloc(alloc_size);
                 if (!buf)
                         return -ENOMEM;
 
                 struct iovec iov[] = {
                         { &attr, sizeof(attr)                           },
-                        { buf,   (size_t) st.st_size - sizeof(attr) + 1 },
+                        { buf,   read_size                              },
                 };
 
                 n = readv(fd, iov, 2);
@@ -103,11 +110,11 @@ int efi_get_variable(
                                 return log_debug_errno(errno, "Reading from '%s' failed: %m", p);
 
                         log_debug("Reading from '%s' failed with EINTR, retrying.", p);
-                } else if ((size_t) n == sizeof(attr) + st.st_size + 1)
+                } else if ((size_t) n == read_limit)
                         /* We need to try again with a bigger buffer, the variable was apparently changed concurrently? */
                         log_debug("EFI variable '%s' larger than expected, retrying.", p);
                 else {
-                        assert((size_t) n < sizeof(attr) + st.st_size + 1);
+                        assert((size_t) n < read_limit);
                         break;
                 }