kernel: sucompat: increase reliability, commonize and micro-optimize tiann #2656

On plain ARMv8.0 devices (A53,A57,A73), strncpy_from_user_nofault() sometimes
fails to copy `filename_user` string correctly. This breaks su ofc, breaking
some apps like Termux (Play Store ver), ZArchiver and Root Explorer.

Apply the susfs patch

This does NOT seem to affect newer ARMv8.2+ CPUs (A75/A76 and newer)

My speculation? ARMv8.0 has weak speculation :)

here we replace `ksu_strncpy_from_user_nofault` with ksu_strncpy_from_user_retry:
- ksu_strncpy_from_user_nofault as fast-path copy
- fallback to access_ok to validate the pointer + strncpy_from_user
- manual null-termination just in case, as strncpy_from_user_nofault also does it
- remove that memset, seems useless as it is an strncpy, not strncat

basically, we retry on pagefualt

for usercopies, its not like were doing
    memset(dest, 0, sizeof(dest));
    strncat(dest, var, bytes);

that memset seems unneeded. instead we use strncpy itself to do proper
error and oob check and null term it after.

as for optimizations
- just return early if unauthorized
- commonized logic
- reduced duplication

Tested on:
- ARMv8.0 A73.a53, A57.a53, A53.a53
- ARMv8.2 A76.a55

Stale: tiann #2656

Co-authored-by: backslashxx <118538522+backslashxx@users.noreply.github.com>
Co-authored-by: ShirkNeko <109797057+ShirkNeko@users.noreply.github.com>
Co-authored-by: rsuntk <rsuntk@yukiprjkt.my.id>
Signed-off-by: ShirkNeko <109797057+ShirkNeko@users.noreply.github.com>

kernel/Makefile

@@ -122,9 +122,6 @@ ccflags-y += -DKSU_COMPAT_HAS_SELINUX_STATE
 endif
 
 # Handle optional backports
-ifeq ($(shell grep -q "strncpy_from_user_nofault" $(srctree)/include/linux/uaccess.h; echo $$?),0)
-ccflags-y += -DKSU_OPTIONAL_STRNCPY
-endif
 ifeq ($(shell grep -q "ssize_t kernel_read" $(srctree)/fs/read_write.c; echo $$?),0)
 ccflags-y += -DKSU_OPTIONAL_KERNEL_READ
 endif