cmd: nvedit: env import can now import only variables passed as parameters

While the `env export` can take as parameters variables to be exported,
`env import` does not have such a mechanism of variable selection.

Let's add the ability to add parameters at the end of the command for
variables to be imported.

Every env variable from the env to be imported passed by parameter to
this command will override the value of the variable in the current env.

If a variable exists in the current env but not in the imported env, if
this variable is passed as a parameter to env import, the variable will
be unset ONLY if the -d option is passed to env import, otherwise the
current value of the variable is kept.

If a variable exists in the imported env, the variable in the current
env will be set to the value of the one from the imported env.

All the remaining variables are left untouched.

As the size parameter of env import is positional but optional, let's
add the possibility to use the sentinel '-' for when we don't want to
give the size parameter (when the env is '\0' terminated) but we pass a
list of variables at the end of the command.

env import addr
env import addr -
env import addr size
env import addr - foo1 foo2
env import addr size foo1 foo2

are all valid.

env import -c addr
env import -c addr -
env import -c addr - foo1 foo2

are all invalid because they don't pass the size parameter required for
checking, while the following are valid.

env import addr size
env import addr size foo1 foo2

Nothing's changed for the other parameters or the overall behaviour.

One of its use case could be to load a secure environment from the
signed U-Boot binary and load only a handful of variables from an
other, unsecure, environment without completely losing control of
U-Boot.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Tested-by: Alex Kiernan <alex.kiernan@gmail.com>
Tested-by: Stephen Warren <swarren@nvidia.com>

cmd/nvedit.c

@@ -1005,8 +1005,11 @@ sep_err:
 
 #ifdef CONFIG_CMD_IMPORTENV
 /*
- * env import [-d] [-t [-r] | -b | -c] addr [size]
- *	-d:	delete existing environment before importing;
+ * env import [-d] [-t [-r] | -b | -c] addr [size] [var ...]
+ *	-d:	delete existing environment before importing if no var is
+ *		passed; if vars are passed, if one var is in the current
+ *		environment but not in the environment at addr, delete var from
+ *		current environment;
  *		otherwise overwrite / append to existing definitions
  *	-t:	assume text format; either "size" must be given or the
  *		text data must be '\0' terminated
@@ -1019,6 +1022,11 @@ sep_err:
  *	addr:	memory address to read from
  *	size:	length of input data; if missing, proper '\0'
  *		termination is mandatory
+ *		if var is set and size should be missing (i.e. '\0'
+ *		termination), set size to '-'
+ *	var...	List of the names of the only variables that get imported from
+ *		the environment at address 'addr'. Without arguments, the whole
+ *		environment gets imported.
  */
 static int do_env_import(cmd_tbl_t *cmdtp, int flag,
 			 int argc, char * const argv[])
@@ -1030,6 +1038,7 @@ static int do_env_import(cmd_tbl_t *cmdtp, int flag,
 	int	fmt = 0;
 	int	del = 0;
 	int	crlf_is_lf = 0;
+	int	wl = 0;
 	size_t	size;
 
 	cmd = *argv;
@@ -1078,9 +1087,9 @@ static int do_env_import(cmd_tbl_t *cmdtp, int flag,
 	addr = simple_strtoul(argv[0], NULL, 16);
 	ptr = map_sysmem(addr, 0);
 
-	if (argc == 2) {
+	if (argc >= 2 && strcmp(argv[1], "-")) {
 		size = simple_strtoul(argv[1], NULL, 16);
-	} else if (argc == 1 && chk) {
+	} else if (chk) {
 		puts("## Error: external checksum format must pass size\n");
 		return CMD_RET_FAILURE;
 	} else {
@@ -1102,6 +1111,9 @@ static int do_env_import(cmd_tbl_t *cmdtp, int flag,
 		printf("## Info: input data size = %zu = 0x%zX\n", size, size);
 	}
 
+	if (argc > 2)
+		wl = 1;
+
 	if (chk) {
 		uint32_t crc;
 		env_t *ep = (env_t *)ptr;
@@ -1116,8 +1128,8 @@ static int do_env_import(cmd_tbl_t *cmdtp, int flag,
 		ptr = (char *)ep->data;
 	}
 
-	if (himport_r(&env_htab, ptr, size, sep, del ? 0 : H_NOCLEAR,
-			crlf_is_lf, 0, NULL) == 0) {
+	if (!himport_r(&env_htab, ptr, size, sep, del ? 0 : H_NOCLEAR,
+		       crlf_is_lf, wl ? argc - 2 : 0, wl ? &argv[2] : NULL)) {
 		pr_err("## Error: Environment import failed: errno = %d\n",
 		       errno);
 		return 1;
@@ -1247,7 +1259,7 @@ static char env_help_text[] =
 #endif
 #endif
 #if defined(CONFIG_CMD_IMPORTENV)
-	"env import [-d] [-t [-r] | -b | -c] addr [size] - import environment\n"
+	"env import [-d] [-t [-r] | -b | -c] addr [size] [var ...] - import environment\n"
 #endif
 	"env print [-a | name ...] - print environment\n"
 #if defined(CONFIG_CMD_RUN)