Re: [PATCH] error-injection: Add prompt for function error injection

[Benjamin Tissoires]

On 12/1/22 22:13, Linus Torvalds wrote:
> On Thu, Dec 1, 2022 at 8:59 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> > The hid-bpf framework depends on it.
>
> Ok, this is completely unacceptably disgusting hack.

[foreword: I have read the other replies, just replying to this one
because it is the explicit ask for a fix]

> That needs fixing.
>
> > Either hid-bpf or bpf core can add
> > "depends on FUNCTION_ERROR_INJECTION"
>
> No, it needs to be narrowed down a lot. Nobody sane wants error
> injection just because they want some random HID thing.
>
> And no, BPF shouldn't need it either.
>
> This needs to be narrowed down to the point where HID can say "I want
> *this* particular call to be able to be a bpf call.

So, would the following be OK? I still have a few concerns I'll explain
after the patch.

---
From 1290561304eb3e48e1e6d727def13c16698a26f1 Mon Sep 17 00:00:00 2001
From: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
Date: Fri, 2 Dec 2022 12:40:29 +0100
Subject: [PATCH] bpf: do not rely on ALLOW_ERROR_INJECTION for fmod_ret

The current way of expressing that a non-bpf kernel component is willing
to accept that bpf programs can be attached to it and that they can change
the return value is to abuse ALLOW_ERROR_INJECTION.
This is debated in the link below, and the result is that it is not a
reasonable thing to do.

An easy fix is to keep the list of valid functions in the BPF verifier
in the same way we keep the non-sleepable ALLOW_ERROR_INJECTION ones.
However, this kind of defeat the point of being able to add bpf APIs in
non-BPF subsystems, so we probably need to rethink that part.


Link: https://lore.kernel.org/all/20221121104403.1545f9b5@xxxxxxxxxxxxxxxxxx/
Suggested-by: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
---
 drivers/hid/bpf/hid_bpf_dispatch.c  |  2 --
 drivers/hid/bpf/hid_bpf_jmp_table.c |  1 -
 kernel/bpf/verifier.c               | 20 +++++++++++++++++++-
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/hid/bpf/hid_bpf_dispatch.c b/drivers/hid/bpf/hid_bpf_dispatch.c
index 3c989e74d249..d1f6a1d4ae60 100644
--- a/drivers/hid/bpf/hid_bpf_dispatch.c
+++ b/drivers/hid/bpf/hid_bpf_dispatch.c
@@ -44,7 +44,6 @@ __weak noinline int hid_bpf_device_event(struct hid_bpf_ctx *ctx)
 {
 	return 0;
 }
-ALLOW_ERROR_INJECTION(hid_bpf_device_event, ERRNO);
 
 u8 *
 dispatch_hid_bpf_device_event(struct hid_device *hdev, enum hid_report_type type, u8 *data,
@@ -105,7 +104,6 @@ __weak noinline int hid_bpf_rdesc_fixup(struct hid_bpf_ctx *ctx)
 {
 	return 0;
 }
-ALLOW_ERROR_INJECTION(hid_bpf_rdesc_fixup, ERRNO);
 
 u8 *call_hid_bpf_rdesc_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size)
 {
diff --git a/drivers/hid/bpf/hid_bpf_jmp_table.c b/drivers/hid/bpf/hid_bpf_jmp_table.c
index 579a6c06906e..207972b028d9 100644
--- a/drivers/hid/bpf/hid_bpf_jmp_table.c
+++ b/drivers/hid/bpf/hid_bpf_jmp_table.c
@@ -103,7 +103,6 @@ __weak noinline int __hid_bpf_tail_call(struct hid_bpf_ctx *ctx)
 {
 	return 0;
 }
-ALLOW_ERROR_INJECTION(__hid_bpf_tail_call, ERRNO);
 
 int hid_bpf_prog_run(struct hid_device *hdev, enum hid_bpf_prog_type type,
 		     struct hid_bpf_ctx_kern *ctx_kern)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 225666307bba..4eac440d659f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -24,6 +24,7 @@
 #include <linux/bpf_lsm.h>
 #include <linux/btf_ids.h>
 #include <linux/poison.h>
+#include <linux/hid_bpf.h>
 
 #include "disasm.h"
 
@@ -14827,6 +14828,20 @@ static int check_non_sleepable_error_inject(u32 btf_id)
 	return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
 }
 
+/* Manually tag fmod_ret functions to not misuse ALLOW_ERROR_INJECTION */
+BTF_SET_START(btf_modify_return)
+#if CONFIG_HID_BPF
+BTF_ID(func, hid_bpf_device_event)
+BTF_ID(func, hid_bpf_rdesc_fixup)
+BTF_ID(func, __hid_bpf_tail_call)
+#endif /* CONFIG_HID_BPF */
+BTF_SET_END(btf_modify_return)
+
+static int check_function_modify_return(u32 btf_id)
+{
+	return btf_id_set_contains(&btf_modify_return, btf_id);
+}
+
 int bpf_check_attach_target(struct bpf_verifier_log *log,
 			    const struct bpf_prog *prog,
 			    const struct bpf_prog *tgt_prog,
@@ -15047,7 +15062,10 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 				bpf_log(log, "can't modify return codes of BPF programs\n");
 				return -EINVAL;
 			}
-			ret = check_attach_modify_return(addr, tname);
+			ret = -EINVAL;
+			if (!check_function_modify_return(btf_id) ||
+			    check_attach_modify_return(addr, tname))
+				ret = 0;
 			if (ret) {
 				bpf_log(log, "%s() is not modifiable\n", tname);
 				return ret;
--
2.38.1
---

While this patch removes the need for ALLOW_ERROR_INJECTION it has a
couple of drawbacks:
- suddenly we lose the nice separation of concerns between bpf core and
its users (HID in my case)
- it would need to be changed in 6.3 simply because of the previous
point, so it is just a temporary fix.

So I am not sure if this would qualify HID-BPF for 6.2. Please speak up.

> Stop this crazy "bpf / hid needs error injection" when that then
> implies a _lot_ more than that, plus is documented to be something
> entirely different anyway.
>
> I realize that HID has mis-used the "we could just use error injection
> here to instead insert random bpf code", but that's a complete hack.

Just to be fair, HID only happens to be the first on the front line for
this particular problem. I was told to use that mis-use because that was
probably what was available, and adding that decorator didn't seem to be
such a big deal to me.

But with a bigger picture in mind, I am happy we get to that point now
before it is merged because I know that behind me there are a few other
people in other subsystems also willing to take advantage of BPF in
their own subsystem. And at Plumbers, everybody was saying: look at the
HID-BPF patch series.

Cheers,
Benjamin

> Plus it seems to happily not even have made it into mainline anyway,
> and only exists in linux-next. Let's head that disgusting hack off at
> the pass.
>
> I'm going to apply Steven's patch, because honestly, we need to fix
> this disgusting mess *before* it gets to mainline, rather than say
> "oh, we already have broken users in next, so let's bend over
> backwards for that".
>
> The code is called "error injection", not "random bpf extension"
>
>                 Linus