mtd: Introduce an expert mode for forensics and debugging purposes

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@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211118114659.1282855-1-miquel.raynal@bootlin.com

drivers/mtd/mtdcore.c

@@ -2370,6 +2370,14 @@ static struct backing_dev_info * __init mtd_bdi_init(const char *name)
 	return ret ? ERR_PTR(ret) : bdi;
 }
 
+char *mtd_expert_analysis_warning =
+	"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);
+bool mtd_expert_analysis_mode;
+EXPORT_SYMBOL_GPL(mtd_expert_analysis_mode);
+
 static struct proc_dir_entry *proc_mtd;
 
 static int __init init_mtd(void)
@@ -2393,6 +2401,8 @@ static int __init init_mtd(void)
 		goto out_procfs;
 
 	dfs_dir_mtd = debugfs_create_dir("mtd", NULL);
+	debugfs_create_bool("expert_analysis_mode", 0600, dfs_dir_mtd,
+			    &mtd_expert_analysis_mode);
 
 	return 0;
 

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 false;
+
 	if (nanddev_bbt_is_initialized(nand)) {
 		unsigned int entry;
 		int status;

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);
 

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;
+
 	switch (res) {
 	case BBT_BLOCK_GOOD:
 		return 0;

include/linux/mtd/mtd.h

@@ -711,4 +711,7 @@ static inline int mtd_is_bitflip_or_eccerr(int err) {
 
 unsigned mtd_mmap_capabilities(struct mtd_info *mtd);
 
+extern char *mtd_expert_analysis_warning;
+extern bool mtd_expert_analysis_mode;
+
 #endif /* __MTD_MTD_H__ */