Neither system calls nor the asm keyword are undefined behavior in the sense that C uses the term. They are, simply put, not covered by the standard at all.
System calls--assuming you're referring to the C prototypes you call--work as normal external function definitions, just having semantics which are defined by the library (i.e., the kernel) and not the C specification itself. The asm keyword is a compiler language extension and is effectively implementation-defined (as C would call it), although compilers today tend to poorly document the actual semantics of their extensions.
System calls--assuming you're referring to the C prototypes you call--work as normal external function definitions, just having semantics which are defined by the library (i.e., the kernel) and not the C specification itself. The asm keyword is a compiler language extension and is effectively implementation-defined (as C would call it), although compilers today tend to poorly document the actual semantics of their extensions.