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bpf: Add cgroup helpers bpf_{get,set}_retval to get/set syscall return value

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The helpers continue to use int for retval because all the hooks
are int-returning rather than long-returning. The return value of
bpf_set_retval is int for future-proofing, in case in the future
there may be errors trying to set the retval.

After the previous patch, if a program rejects a syscall by
returning 0, an -EPERM will be generated no matter if the retval
is already set to -err. This patch change it being forced only if
retval is not -err. This is because we want to support, for
example, invoking bpf_set_retval(-EINVAL) and return 0, and have
the syscall return value be -EINVAL not -EPERM.

For BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY, the prior behavior is
that, if the return value is NET_XMIT_DROP, the packet is silently
dropped. We preserve this behavior for backward compatibility
reasons, so even if an errno is set, the errno does not return to
caller. However, setting a non-err to retval cannot propagate so
this is not allowed and we return a -EFAULT in that case.

Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/b4013fd5d16bed0b01977c1fafdeae12e1de61fb.1639619851.git.zhuyifei@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
YiFei Zhu
2021-12-16 02:04:27 +00:00
committed by Andrii Nakryiko
parent 75c7c722f5
commit ab022c8eb4
1 changed files with 18 additions and 0 deletions
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18
include/uapi/linux/bpf.h
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@@ -5033,6 +5033,22 @@ union bpf_attr {
*
* Return
* The number of arguments of the traced function.
*
* int bpf_get_retval(void)
* Description
* Get the syscall's return value that will be returned to userspace.
*
* This helper is currently supported by cgroup programs only.
* Return
* The syscall's return value.
*
* int bpf_set_retval(int retval)
* Description
* Set the syscall's return value that will be returned to userspace.
*
* This helper is currently supported by cgroup programs only.
* Return
* 0 on success, or a negative error in case of failure.
*/
#define __BPF_FUNC_MAPPER(FN) \
FN(unspec), \
@@ -5221,6 +5237,8 @@ union bpf_attr {
FN(get_func_arg), \
FN(get_func_ret), \
FN(get_func_arg_cnt), \
FN(get_retval), \
FN(set_retval), \
/* */
/* integer value in 'imm' field of BPF_CALL instruction selects which helper