[rtems commit] Fix a UB when fdt_get_string return null

[Sebastian Huber]

Module:    rtems
Branch:    master
Commit:    6f79435915cfe57c64964ff85c234b68718980c9
Changeset: http://git.rtems.org/rtems/commit/?id=6f79435915cfe57c64964ff85c234b68718980c9

Author:    LoveSy <shana at zju.edu.cn>
Date:      Wed Dec 15 17:30:11 2021 +0800

Fix a UB when fdt_get_string return null

When fdt_get_string return null, `namep` is not correctly reset.
>From the document of `fdt_getprop_by_offset`, the parameter `namep` will
be always overwritten (that is, it will be overwritten without exception
of error occurance).

As for the caller (like
https://github.com/topjohnwu/Magisk/blob/e097c097feb881f6097b6d1dc346f310bc92f5d6/native/jni/magiskboot/dtb.cpp#L42),
the code may be like:
```cpp
size_t size;
const char *name;
auto *value = fdt_getprop_by_offset(fdt, prop, &name, &size);
```
and if `value == nullptr`, `size` is also be overwritten correctly but
`name` is not, which is quite inconsistent.

This commit makes sure `name` and `size` behavior consistently (reset to
reasonable value) when error occurs.

Signed-off-by: LoveSy <shana at zju.edu.cn>
Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

---

 cpukit/dtc/libfdt/fdt_ro.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cpukit/dtc/libfdt/fdt_ro.c b/cpukit/dtc/libfdt/fdt_ro.c
index 17584da257..9f6c551a22 100644
--- a/cpukit/dtc/libfdt/fdt_ro.c
+++ b/cpukit/dtc/libfdt/fdt_ro.c
@@ -481,12 +481,12 @@ const void *fdt_getprop_by_offset(const void *fdt, int offset,
 		if (!can_assume(VALID_INPUT)) {
 			name = fdt_get_string(fdt, fdt32_ld_(&prop->nameoff),
 					      &namelen);
+			*namep = name;
 			if (!name) {
 				if (lenp)
 					*lenp = namelen;
 				return NULL;
 			}
-			*namep = name;
 		} else {
 			*namep = fdt_string(fdt, fdt32_ld_(&prop->nameoff));
 		}