bpf: remove explicit handling of 0 for arg2 in bpf_probe_read

gianlucaborello · borkmann · commit eb33f2cca49e · 2017-11-22T21:40:54.000+01:00
Commit 9c019e2 ("bpf: change helper bpf_probe_read arg2 type to ARG_CONST_SIZE_OR_ZERO") changed arg2 type to ARG_CONST_SIZE_OR_ZERO to simplify writing bpf programs by taking advantage of the new semantics introduced for ARG_CONST_SIZE_OR_ZERO which allows <!NULL, 0> arguments. In order to prevent the helper from actually passing a NULL pointer to probe_kernel_read, which can happen when <NULL, 0> is passed to the helper, the commit also introduced an explicit check against size == 0. After the recent introduction of the ARG_PTR_TO_MEM_OR_NULL type, bpf_probe_read can not receive a pair of <NULL, 0> arguments anymore, thus the check is not needed anymore and can be removed, since probe_kernel_read can correctly handle a <!NULL, 0> call. This also fixes the semantics of the helper before it gets officially released and bpf programs start relying on this check. Fixes: 9c019e2 ("bpf: change helper bpf_probe_read arg2 type to ARG_CONST_SIZE_OR_ZERO") Signed-off-by: Gianluca Borello <g.borello@gmail.com> Acked-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Daniel Borkmann <daniel@iogearbox.net> Acked-by: Yonghong Song <yhs@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
@@ -78,16 +78,12 @@ EXPORT_SYMBOL_GPL(trace_call_bpf);
 
 BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
 {
-	int ret = 0;
-
-	if (unlikely(size == 0))
-		goto out;
+	int ret;
 
 	ret = probe_kernel_read(dst, unsafe_ptr, size);
 	if (unlikely(ret < 0))
 		memset(dst, 0, size);
 
- out:
 	return ret;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -78,16 +78,12 @@ EXPORT_SYMBOL_GPL(trace_call_bpf);`
`78`	`78`
`79`	`79`	`BPF_CALL_3(bpf_probe_read, void , dst, u32, size, const void , unsafe_ptr)`
`80`	`80`	`{`
`81`		`- int ret = 0;`
`82`		`-`
`83`		`- if (unlikely(size == 0))`
`84`		`- goto out;`
	`81`	`+ int ret;`
`85`	`82`
`86`	`83`	`ret = probe_kernel_read(dst, unsafe_ptr, size);`
`87`	`84`	`if (unlikely(ret < 0))`
`88`	`85`	`memset(dst, 0, size);`
`89`	`86`
`90`		`- out:`
`91`	`87`	`return ret;`
`92`	`88`	`}`
`93`	`89`