Re: bio_check_ro @ blk-core.c

[Pascal <patatetom@xxxxxxxxx>]

hi Emil,

in view of your answer and after rereading my email, I realize that I was confused in my request.
here it is, I hope, more clearly reformulated :-)

first of all, I use ArchLinux to, from time to time, compile the slightly modified LTS kernel, and this from PKGBUILD provided by ArchLinux at some point.

some technologies such as LVM do not take into account the read-only applied on a block device.
see the two links provided in the previous exchanges for more details...

until now, I recompiled the kernel by applying a slight modification to the bio_check_ro function present in the blk-core.c source file.
the last time I made this modification was on the Linux-LTS-5.10.19 kernel :
(https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/block/blk-core.c?h=v5.10.19)

$ diff   -u   5.10.19.original/blk-core.c   5.10.19.me/blk-core.c
--- 5.10.19.original/blk-core.c 2023-03-15 13:44:20.176929833 +0100
+++ 5.10.19.me/blk-core.c 2023-03-15 13:44:02.353596114 +0100
@@ -706,7 +706,7 @@
        "Trying to write to read-only block-device %s (partno %d)\n",
  bio_devname(bio, b), part->partno);
  /* Older lvm-tools actually trigger this */
- return false;
+ return true;
  }
 
  return false;

the compilation of the modified LTS 5.10.19 kernel went well and the correction seems to do the job...

since this last time (2022/01), the source file blk-core.c has been modified a lot and the bio_check_ro function is part of these modifications :

(https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/block/blk-core.c?h=v6.1.15)

$ diff   -u   5.10.19.original/blk-core.c   6.1.15.original/blk-core.c
--- 5.10.19.original/blk-core.c 2023-03-15 13:44:20.176929833 +0100
+++ 6.1.15.original/blk-core.c 2023-03-15 13:50:36.560271323 +0100
@@ -14,11 +14,10 @@
  */
 #include <linux/kernel.h>
 #include <linux/module.h>
-#include <linux/backing-dev.h>
 #include <linux/bio.h>
 #include <linux/blkdev.h>
-#include <linux/blk-mq.h>
...
@@ -681,40 +483,22 @@
 }
 
 late_initcall(fail_make_request_debugfs);
-
-#else /* CONFIG_FAIL_MAKE_REQUEST */
-
-static inline bool should_fail_request(struct hd_struct *part,
-                   unsigned int bytes)
-{
-   return false;
-}
-
 #endif /* CONFIG_FAIL_MAKE_REQUEST */
 
-static inline bool bio_check_ro(struct bio *bio, struct hd_struct *part)
+static inline void bio_check_ro(struct bio *bio)
 {
-   const int op = bio_op(bio);
-
-   if (part->policy && op_is_write(op)) {
-       char b[BDEVNAME_SIZE];
-
+   if (op_is_write(bio_op(bio)) && bdev_read_only(bio->bi_bdev)) {
        if (op_is_flush(bio->bi_opf) && !bio_sectors(bio))
-           return false;
-
-       WARN_ONCE(1,
-              "Trying to write to read-only block-device %s (partno %d)\n",
-           bio_devname(bio, b), part->partno);
+           return;
+       pr_warn("Trying to write to read-only block-device %pg\n",
+           bio->bi_bdev);
        /* Older lvm-tools actually trigger this */
-       return false;
    }
-
-   return false;
 }
...

when I introduce my little modification (see diff below) in the code, makepkg fails to compile with the error "return with a value in function returning void" (see makepkg below) :

$ diff   -u   6.1.15.original/blk-core.c   6.1.15.me/blk-core.c

--- 6.1.15.original/blk-core.c 2023-03-15 13:50:36.560271323 +0100
+++ 6.1.15.me/blk-core.c 2023-03-15 13:56:15.246945330 +0100
@@ -493,6 +493,7 @@
  pr_warn("Trying to write to read-only block-device %pg\n",
  bio->bi_bdev);
  /* Older lvm-tools actually trigger this */
+ return true;
  }
 }

$ makepkg
...
  CC      block/blk-core.o
block/blk-core.c: In function 'bio_check_ro':
block/blk-core.c:496:24: error: 'return' with a value, in function returning void [-Werror=return-type]
  496 |                 return true;
      |                        ^~~~
block/blk-core.c:488:20: note: declared here
  488 | static inline void bio_check_ro(struct bio *bio)
      |                    ^~~~~~~~~~~~
cc1: some warnings being treated as errors
make[2]: *** [scripts/Makefile.build:250: block/blk-core.o] Error 1
make[1]: *** [scripts/Makefile.build:500: block] Error 2
make: *** [Makefile:2005: .] Error 2

so, how to modify the current code of the bio_check_ro function to get the desired result (eg. writes KO on RO blockdevice) ?

with the changes in the blk-core.c source code since version 5.10.19, is it still necessary to tweak the bio_check_ro function to disallow technologies that ignore the read-only block?

regards, lacsaP.

Le mer. 15 mars 2023 à 12:37, Emil Velikov <emil.l.velikov@xxxxxxxxx> a écrit :

Greetings Pascal,

Couple of suggestions from the peanut gallery, Take them with a heavy
pinch of salt:
 - Is the issue happening with upstream code from kernel.org?
 - Consider mentioning the commit sha (and URL, if it is missing from
kernel.org) in the email
 - Is "intervened" the right word here - the Cambridge dictionary
defines it as "to intentionally become involved in a difficult
situation in order to improve it or prevent it from getting worse"
 - Are you contacting a developer only? Have you considered adding the
subsystem maintainer and mailing list in the CC list -
scripts/get_maintainer.pl will give you those
 - Have you considered opening a bug report, or better yet sending a
patch? Patch does not have to be perfect and if you have doubts you
can mention those in the email/cover-letter.

Hope that helps
Emil

[1] https://dictionary.cambridge.org/dictionary/english/intervene

On Wed, 15 Mar 2023 at 08:42, Pascal <patatetom@xxxxxxxxx> wrote:
>
> hi,
>
> I come to you for lack of feedback (I think the Linux kernel developers have other cats to whip :-))
> would one of you have the answer or a track to follow concerning the question below ?
> the encountered compilation error is behind the forwarded email.
>
> regards, lacsaP.
>
> ---------- Forwarded message ---------
> De : Pascal <patatetom@xxxxxxxxx>
> Date: mer. 8 mars 2023 à 14:09
> Subject: bio_check_ro @ blk-core.c
>
> hi,
>
> I'm addressing you because you intervened (commit) in the function bio_check_ro @ blk-core.c @ Linux-LTS-6.1.15.
> the last time I intervened on this file (@ Linux-LTS-5.10.19 for personal use), it was to replace "return false;" by "return true;", which theoretically should prevent the possible writing on a device locked in read-only mode (see here or here).
> with @ Linux-LTS-6.1.15, if I insert "return true;", I now have a compilation error.
> in your opinion, is there still a need to "fix" blk-core.c to prevent writing to a read-only locked device and if so, can you help me implement this fix?
>
> regards, lacsaP.
> ---------- End forwarded message ---------
>
>   SYNC    include/config/auto.conf
>   CC      arch/x86/kernel/asm-offsets.s
>   CALL    scripts/checksyscalls.sh
>   DESCEND objtool
>   DESCEND bpf/resolve_btfids
>   CC      block/bdev.o
>   CC      block/fops.o
>   CC      block/bio.o
>   CC      block/elevator.o
>   CC      block/blk-core.o
> block/blk-core.c: In function 'bio_check_ro':
> block/blk-core.c:496:24: error: 'return' with a value, in function returning void [-Werror=return-type]
>   496 |                 return true;
>       |                        ^~~~
> block/blk-core.c:488:20: note: declared here
>   488 | static inline void bio_check_ro(struct bio *bio)
>       |                    ^~~~~~~~~~~~
> cc1: some warnings being treated as errors
> make[2]: *** [scripts/Makefile.build:250: block/blk-core.o] Error 1
> make[1]: *** [scripts/Makefile.build:500: block] Error 2
> make: *** [Makefile:2005: .] Error 2