This patch-set implements the skeleton for a new sys_indirect() system call.
The reason for such system call would be to avoid the proliferation of new
system calls, introduced to only wrap old ones by setting/unsetting a given
context before/after the real system call.
The internal operation of sys_indirect() would be (pseudo-code):
for_each_ctx(wrap_ctx, &user_ctxs) {
save_prev_ctx(&saved_chain, tsk->xxx);
tsk->xxx = wrap_ctx;
}
err = call_system_call(nr, params);
for_each_saved_ctx(ctx, &saved_chain) {
restore_ctx(tsk->xxx, ctx);
}
return err;
To made sys_indirect() decently flexible, and to avoid a sys_indirect2()
anytime soon, a simple "flags" parameter is clearly not sufficent to be
passed down with it.
To be flexible we need to allow 1) more than a simple "flags" to be passed
down 2) more than one context possibly to be set before the real system
call. Also, the data structures passed down as new-context-to-be-set should
be compat-free, in order to simplify the compat code.
The prototype for sys_indirect() is:
struct indirect_ctx {
__u32 ctx;
};
long sys_indirect(unsigned int nr,
const struct indirect_ctx **ctxs,
unsigned int nctxs,
const unsigned long *params);
The "struct indirect_ctx" is a stub structure that is supposed to be the
base for all the context set/unset operations.
An example usage for userspace POV (referring to the example in the next
patches) is:
[include/linux/indirect.h]
#define SYSIND_CTX_OPENFLAGS 0
struct sysind_ctx_OPENFLAGS {
__u32 ctx;
__u32 flags;
};
[userspace]
struct sysind_ctx_OPENFLAGS octx;
struct indirect_ctx *ctxs[1];
unsigned long params[6];
octx.ctx = SYSIND_CTX_OPENFLAGS;
octx.flags = O_CLOEXEC;
ctxs[0] = (struct indirect_ctx *) &octx;
params[0] = domain;
params[1] = type;
params[2] = proto;
res = indirect(__NR_socket, ctxs, 1, params);
The sys_indirect() system call requires a small arch-dependent asm function
call_syscall() in order to call a system call by passing call number and
parameters (compat_call_syscall() is also needed for archs having 32 bit compat).
I currently implemented them for i386 and x86-64.
The following patches builds but are totally untested at the moment.
Comments?
- Davide
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