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. 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. 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)
