Messages in this thread | | | Date | Sun, 28 Jan 2018 20:21:28 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 3/3] syscalls: Add a bit of documentation to __SYSCALL_DEFINE |
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* Andy Lutomirski <luto@kernel.org> wrote:
> __SYSCALL_DEFINE is rather magical. Add a bit of documentation. > > Signed-off-by: Andy Lutomirski <luto@kernel.org> > --- > include/linux/syscalls.h | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index a78186d826d7..d3f244a447c5 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -207,6 +207,16 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event) > __SYSCALL_DEFINEx(x, sname, __VA_ARGS__) > > #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__) > + > +/* > + * For a syscall like long foo(void *a, long long b), this defines: > + * > + * static inline long SYSC_foo(void *a, long long b): the actual code > + * > + * asmlinkage long SyS_foo(long a, long long b): wrapper that calls SYSC_foo > + * > + * asmlinkage long sys_foo(void *a, long long b): alias of SyS_foo > + */
Nit, would it be more readable to write this as pseudo-code, i.e. something like:
/* * For a syscall like long foo(void *a, long long b), this defines: * * static inline long SYSC_foo(void *a, long long b) { /* the actual code following */ } * * asmlinkage long SyS_foo(long a, long long b) { /* wrapper that calls SYSC_foo() */ } * asmlinkage long sys_foo(void *a, long long b); /* as GCC alias of SyS_foo() */ */
Also, I wanted to suggest to also document that in practice the three methods map to the very same function in the end, with the SYSC_ variant being eliminated due to inlining - but when double checking an x86-64 defconfig that does not appear to be so for all system calls:
triton:~/tip> grep accept4 System.map ffffffff8170d710 t SYSC_accept4 ffffffff8170f940 T SyS_accept4 ffffffff8170f940 T sys_accept4
While for others there's just 2:
triton:~/tip> grep sched_getattr System.map ffffffff8107b9a0 T SyS_sched_getattr ffffffff8107b9a0 T sys_sched_getattr
The only difference appears to be that accept4() is called internally within the kernel, by socketcall:
SYSCALL_DEFINE3(accept, int, fd, struct sockaddr __user *, upeer_sockaddr, int __user *, upeer_addrlen) { return sys_accept4(fd, upeer_sockaddr, upeer_addrlen, 0); }
But why does that result in SYSC_accept4() being a different symbol?
The difference between the two appears to be rather dumb as well:
ffffffff8170f940 <SyS_accept4>: ffffffff8170f940: e9 cb dd ff ff jmpq ffffffff8170d710 <SYSC_accept4>
Using GCC 7.2.0.
What am I missing?
Thanks,
Ingo
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