Hello Walter, On 2021-09-17, Walter Stoll <Walter.Stoll@xxxxxxxxxx> wrote: > Effect observed > --------------- > > We want to store kernel oops logs on a NAND flash partition by means of the > MTD Oops/Panic console logger/driver. However, no log is generated. > > We observed the effect with kernel version v5.4.138-rt62. However, we think > that the most recent kernel exhibits the same behavior because the structure of > the sources in question (see below) did not change. This problem does not exist in v5.10-rt. The kmsg_dump API was changed (later in mainline as well) to avoid this issue. For v5.10-rt the change was buried in the all-in-one commit: https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git/commit/?h=v5.10-rt&id=9835363e31e75d3287a81c6fc1b22c5c2b43d277 And later. the API change for mainline (5.12): https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f9f3f02db98bbe678a8e57fe9432b196174744a3 > Root cause > ---------- > > Further investigations revealed, than the issue is due to the realtime adaption > of the kmsg_dump() function. > > Non realtime version of kmsg_dump(): > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/printk/printk.c?h=linux-5.4.y#n3149 > > Realtime version of kmsg_dump(): > https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git/tree/kernel/printk/printk.c?h=v5.4-rt#n2904 > > In our case, the the mtd_panic_write() function returns with error when the > kernel panics. As a result, no log is created. Call chain: > ... panic() -> kmsg_dump() -> mtdoops_do_dump() -> mtdoops_write() > mtd_panic_write() > > Note that within the kmsg_dump() function, a locally create kmsg_dumper object > is passed to the dumper_local.dump(&dumper_local, reason) call. However the > callee mtdoops_do_dump() function expects this object being embedded in a > mtdoops_context object, see > https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git/tree/drivers/mtd/mtdoops.c?h=v5.4-rt#n272 > > This is obviously not the case, therefore the error. Note that this could even > led to a security issue. > > > Bug fix proposal > ---------------- > > We fixed the bug locally by applying the patch below. Note that we did not > modify the kmsg_dump() function. Instead we modified the mtdoops_do_dump() > function which seems for us the cleaner way. I agree that this is a better fix, rather than backporting the newer API. For mtdoops, the only function registering a dumper is mtdoops_notify_add() and it also directly references the global context with: struct mtdoops_context *cxt = &oops_cxt; Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx> > diff --git a/drivers/mtd/mtdoops.c b/drivers/mtd/mtdoops.c > index 774970bfcf85..317ecf47fce8 100644 > --- a/drivers/mtd/mtdoops.c > +++ b/drivers/mtd/mtdoops.c > @@ -269,8 +269,7 @@ static void find_next_position(struct mtdoops_context *cxt) > static void mtdoops_do_dump(struct kmsg_dumper *dumper, > enum kmsg_dump_reason reason) > { > - struct mtdoops_context *cxt = container_of(dumper, > - struct mtdoops_context, dump); > + struct mtdoops_context *cxt = &oops_cxt; > > /* Only dump oopses if dump_oops is set */ > if (reason == KMSG_DUMP_OOPS && !dump_oops) Thanks for your work on this! John Ogness
