What do router_defaults and Nvrams refer to?and how to run test.c?

[1163307648]

Hello,I want to know how to emulate a hardware NVRAM peripheral and i get two questions after reading the libnvram's code:
1: in config.h
#def NVRAM_DEFAULTS_PATH
TABLE (router_defaults)
TABLE (Nvrams)

What do router_defaults and Nvrams refer to? I did not find their definitions.

2: how to run test.c in libnvram

Thanks for the answer

[ddcc]

The prototype for the TABLE macro is on nvram.c#L25. Essentially, some firmware have a global symbol that provide a key-value pair list of default values, which is why it is declared as an external weak symbol. This is also discussed on pg. 7 of the paper.

The test file isn't super usable, but you can compile it as follows. It's a little invasive at runtime though, it'll assume that the mount point /nvram exists, and that it can mount a tmpfs onto it.

gcc test.c -o test -L`pwd` -lnvram -Wl,-rpath=`pwd`

[cq674350529]

Recently, I have used the libnvram.so in qemu user mode, and it works well. Thanks!

Also, I came across an issue: some values in NVRAM_DEFAULTS didn't work as expected. As you (@ddcc) mentioned above, the router_defaults and Nvrams are global symbols used to provide default values.

libnvram/nvram.c

Lines 571 to 595 in 68ad8e9

	int nvram_set_default(void) {
	int ret = nvram_set_default_builtin();
	PRINT_MSG("Loading built-in default values = %d!\n", ret);
	#define NATIVE(a, b) \
	if (!system(a)) { \
	PRINT_MSG("Executing native call to built-in function: %s (%p) = %d!\n", #b, b, b); \
	}
	#define TABLE(a) \
	PRINT_MSG("Checking for symbol \"%s\"...\n", #a); \
	if (a) { \
	PRINT_MSG("Loading from native built-in table: %s (%p) = %d!\n", #a, a, nvram_set_default_table(a)); \
	}
	#define PATH(a) \
	if (!access(a, R_OK)) { \
	PRINT_MSG("Loading from default configuration file: %s = %d!\n", a, foreach_nvram_from(a, (void (*)(const char *, const char *, void *)) nvram_set, NULL)); \
	}
	NVRAM_DEFAULTS_PATH
	#undef PATH
	#undef NATIVE
	#undef TABLE

The order of call to nvram_set_default_* are: nvram_set_default_builtin(), nvram_set_default_table(a). That is, the values in NVRAM_DEFAULTS will be set first, then the values in router_defaults and Nvrams. So there may be a case the value in NVRAM_DEFAULTS will be overwritten by those in router_defaults and Nvrams.

In my case, I changed the order of call to nvram_set_default_* , shown as follows. So the values in NVRAM_DEFAULTS will work as expected.

PS: I didn't test in in qemu system mode, so didn't know if it had any side effect.

#define NATIVE(a, b) \
    if (!system(a)) { \
        PRINT_MSG("Executing native call to built-in function: %s (%p) = %d!\n", #b, b, b); \
    }

#define TABLE(a) \
    PRINT_MSG("Checking for symbol \"%s\"...\n", #a); \
    if (a) { \
        PRINT_MSG("Loading from native built-in table: %s (%p) = %d!\n", #a, a, nvram_set_default_table(a)); \
    }

#define PATH(a) \
    if (!access(a, R_OK)) { \
        PRINT_MSG("Loading from default configuration file: %s = %d!\n", a, foreach_nvram_from(a, (void (*)(const char *, const char *, void *)) nvram_set, NULL)); \
    }

    NVRAM_DEFAULTS_PATH
#undef PATH
#undef NATIVE
#undef TABLE

    // move it to here
    PRINT_MSG("Loading built-in default values = %d!\n", nvram_set_default_builtin());

[ddcc]

Yeah I think the assumption behind that ordering was that the device-specific default symbols should be preferred over any generic pre-defined defaults from the config file. However, they should all be less preferable than the overrides, which are set last.