[PATCH] bpf: whitelist syscalls for error injection

[Howard McLauchlan <hmclauchlan@fb.com>]

Error injection is a useful mechanism to fail arbitrary kernel
functions. However, it is often hard to guarantee an error propagates
appropriately to user space programs. By injecting into syscalls, we can
return arbitrary values to user space directly; this increases
flexibility and robustness in testing, allowing us to test user space
error paths effectively.

The following script, for example, fails calls to sys_open() from a
given pid:

from bcc import BPF
from sys import argv

pid = argv[1]

prog = r"""

int kprobe__SyS_open(struct pt_regs *ctx, const char *pathname, int flags)
{
    u32 pid = bpf_get_current_pid_tgid();
    if (pid == %s)
        bpf_override_return(ctx, -ENOENT);
    return 0;
}
""" % pid

b = BPF(text = prog)
while 1:
    b.perf_buffer_poll()

This patch whitelists all syscalls defined with SYSCALL_DEFINE for error
injection.

Signed-off-by: Howard McLauchlan <hmclauchlan@fb.com>
---
based on 4.16-rc5
 include/linux/syscalls.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a78186d826d7..e8c6d63ace78 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -191,6 +191,8 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
 
 #define SYSCALL_DEFINE0(sname)					\
 	SYSCALL_METADATA(_##sname, 0);				\
+	asmlinkage long sys_##sname(void);			\
+	ALLOW_ERROR_INJECTION(sys_##sname, ERRNO);		\
 	asmlinkage long sys_##sname(void)
 
 #define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)
@@ -210,6 +212,7 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
 #define __SYSCALL_DEFINEx(x, name, ...)					\
 	asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))	\
 		__attribute__((alias(__stringify(SyS##name))));		\
+	ALLOW_ERROR_INJECTION(sys##name, ERRNO);			\
 	static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__));	\
 	asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__));	\
 	asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))	\
-- 
2.14.1