ttyname_r() passed the caller buffer straight to fcntl(F_GETPATH)
whenever buflen >= TTY_NAME_MAX, but every FIOC_FILEPATH handler
writes the path bounded by PATH_MAX and ignores the caller buffer
size. A tty registered under a nested /dev path, or reached through
rpmsgfs, has a path longer than TTY_NAME_MAX and overwrote the caller
buffer, silently corrupting memory behind a zero return code. The
small-buffer branch had the same defect against its own stack local
char name[TTY_NAME_MAX]. Gate the direct write on PATH_MAX instead
and stage the path through a PATH_MAX path buffer obtained via
lib_get_tempbuffer(), returning ERANGE when it does not fit.
Verified on sim:nsh with a test driver registered at an 85-character
tty path: pre-fix, ttyname_r(buf, TTY_NAME_MAX) returned 0 and
smashed the canaries behind the buffer, and the small-buffer branch
panicked; post-fix both cases return ERANGE with the canaries intact.
Assisted-by: Claude Code (glm-5.3) <claude@anthropic.com>
Signed-off-by: Junbo Zheng <zhengjunbo1@xiaomi.com>
82b2f1993b