[PATCH] mtd: Introduce an expert mode for forensics and debugging purposes

[Miquel Raynal]

Hi Geert,

geert at linux-m68k.org wrote on Mon, 10 Jan 2022 14:15:27 +0100:

> Hi Miquel,
> 
> On Thu, Nov 18, 2021 at 12:47 PM Miquel Raynal
> <miquel.raynal at bootlin.com> wrote:
> > When developping NAND controller drivers or when debugging filesystem
> > corruptions, it is quite common to need hacking locally into the
> > MTD/NAND core in order to get access to the content of the bad
> > blocks. Instead of having multiple implementations out there let's
> > provide a simple yet effective specific MTD-wide debugfs entry to fully
> > disable these checks on purpose.
> >
> > A warning is added to inform the user when this mode gets enabled.
> >
> > Signed-off-by: Miquel Raynal <miquel.raynal at bootlin.com>  
> 
> Thanks for your patch, which is now commit 67b967ddd93d0ed5 ("mtd:
> Introduce an expert mode for forensics and debugging purposes")
> in mtd/next.

Thanks for reviewing! Unfortunately I've sent the MTD pull-request to
Linus this morning so I'll have to address this in subsequent
commits.

> > --- a/drivers/mtd/mtdcore.c
> > +++ b/drivers/mtd/mtdcore.c
> > @@ -2365,6 +2365,14 @@ static struct backing_dev_info * __init mtd_bdi_init(const char *name)
> >         return ret ? ERR_PTR(ret) : bdi;
> >  }
> >
> > +char *mtd_expert_analysis_warning =  
> 
> const

With the function you propose, I'll even have to turn it static.

> > +       "Bad block checks have been entirely disabled.\n"
> > +       "This is only reserved for post-mortem forensics and debug purposes.\n"
> > +       "Never enable this mode if you do not know what you are doing!\n";
> > +EXPORT_SYMBOL_GPL(mtd_expert_analysis_warning);  
> 
> Shouldn't this depend on CONFIG_DEBUG_FS?

I haven't received any robot warnings about this (generally speaking
random configs are quite efficient to trigger those errors) so I
believe it is safe? But I'll double check.

> > +bool mtd_expert_analysis_mode;
> > +EXPORT_SYMBOL_GPL(mtd_expert_analysis_mode);  
> 
> Do you really need to export these two symbols?
> 
> > +
> >  static struct proc_dir_entry *proc_mtd;
> >
> >  static int __init init_mtd(void)  
> =
> > --- a/drivers/mtd/nand/core.c
> > +++ b/drivers/mtd/nand/core.c
> > @@ -21,6 +21,9 @@
> >   */
> >  bool nanddev_isbad(struct nand_device *nand, const struct nand_pos *pos)
> >  {
> > +       if (WARN_ONCE(mtd_expert_analysis_mode, mtd_expert_analysis_warning))
> > +               return 0;
> > +
> >         if (nanddev_bbt_is_initialized(nand)) {
> >                 unsigned int entry;
> >                 int status;
> > diff --git a/drivers/mtd/nand/raw/nand_base.c b/drivers/mtd/nand/raw/nand_base.c
> > index 3d6c6e880520..b3a9bc08b4bb 100644
> > --- a/drivers/mtd/nand/raw/nand_base.c
> > +++ b/drivers/mtd/nand/raw/nand_base.c
> > @@ -321,6 +321,9 @@ static int nand_isbad_bbm(struct nand_chip *chip, loff_t ofs)
> >         if (nand_region_is_secured(chip, ofs, mtd->erasesize))
> >                 return -EIO;
> >
> > +       if (WARN_ONCE(mtd_expert_analysis_mode, mtd_expert_analysis_warning))
> > +               return 0;
> > +
> >         if (chip->legacy.block_bad)
> >                 return chip->legacy.block_bad(chip, ofs);
> >
> > diff --git a/drivers/mtd/nand/raw/nand_bbt.c b/drivers/mtd/nand/raw/nand_bbt.c
> > index b7ad030225f8..ab630af3a309 100644
> > --- a/drivers/mtd/nand/raw/nand_bbt.c
> > +++ b/drivers/mtd/nand/raw/nand_bbt.c
> > @@ -1455,6 +1455,9 @@ int nand_isbad_bbt(struct nand_chip *this, loff_t offs, int allowbbt)
> >         pr_debug("nand_isbad_bbt(): bbt info for offs 0x%08x: (block %d) 0x%02x\n",
> >                  (unsigned int)offs, block, res);
> >
> > +       if (WARN_ONCE(mtd_expert_analysis_mode, mtd_expert_analysis_warning))
> > +               return 0;
> > +  
> 
> These are all the same.
> 
> What about letting drivers/mtd/mtdcore.c export a simple function
> mtd_check_expert_analysis_mode() that calls the WARN_ONCE(...) if
> CONFIG_DEBUG_FS=y, else providing a dummy?
> The backtrace will identify the caller anyway.

Yep that's a good idea.

Thanks,
Miquèl