[PATCH 1/3] kstrtox: introduce a safer version of memparse()

Qu Wenruo
2023-12-23T09:58:39Z

[BUGS]
Function memparse() lacks error handling:

- If no valid number string at all
In that case @retptr would just be updated and return value would be
zero.

- No overflown detection
This applies to both the number string part, and the suffixes part.
And since we have no way to indicate errors, we can get weird results
like:

"25E" -> 10376293541461622784 (9E)

This is due to the fact that for "E" suffix, there is only 4 bits
left, and 25 with 60 bits left shift would lead to overflow.

[CAUSE]
The root cause is already mentioned in the comments of the function, the
usage of simple_strtoull() is the source of evil.
Furthermore the function prototype is no good either, just returning an
unsigned long long gives us no way to indicate an error.

[FIX]
Due to the prototype limits, we can not have a drop-in replacement for
memparse().

This patch can only help by introduce a new helper, memparse_safe(), and
mark the old memparse() deprecated.

The new memparse_safe() has the following improvement:

- Invalid string detection
If no number string can be detected at all, -EINVAL would be returned.

- Better overflow detection
Both the string part and the extra left shift would have overflow
detection.
Any overflow would result -ERANGE.

- Safer default suffix selection
The helper allows the caller to choose the suffixes that they want to
use.
But only "KMGTP" are recommended by default since the "E" leaves only
4 bits before overflow.
For those callers really know what they are doing, they can still
manually to include all suffixes.

Due to the prototype change, callers should migrate to the new one and
change their code and add extra error handling.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
include/linux/kernel.h | 8 +++-
include/linux/kstrtox.h | 15 +++++++
lib/cmdline.c | 5 ++-
lib/kstrtox.c | 96 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 122 insertions(+), 2 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index d9ad21058eed..b1b6da60ea43 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -201,7 +201,13 @@ void do_exit(long error_code) __noreturn;

extern int get_option(char **str, int *pint);
extern char *get_options(const char *str, int nints, int *ints);
-extern unsigned long long memparse(const char *ptr, char **retptr);
+
+/*
+ * DEPRECATED, lack of any kind of error handling.
+ *
+ * Use memparse_safe() from lib/kstrtox.c instead.
+ */
+extern __deprecated unsigned long long memparse(const char *ptr, char **retptr);
extern bool parse_option_str(const char *str, const char *option);
extern char *next_arg(char *args, char **param, char **val);

diff --git a/include/linux/kstrtox.h b/include/linux/kstrtox.h
index 7fcf29a4e0de..53a1e059dd31 100644
--- a/include/linux/kstrtox.h
+++ b/include/linux/kstrtox.h
@@ -9,6 +9,21 @@
int __must_check _kstrtoul(const char *s, unsigned int base, unsigned long *res);
int __must_check _kstrtol(const char *s, unsigned int base, long *res);

+enum memparse_suffix {
+ MEMPARSE_SUFFIX_K = 1 << 0,
+ MEMPARSE_SUFFIX_M = 1 << 1,
+ MEMPARSE_SUFFIX_G = 1 << 2,
+ MEMPARSE_SUFFIX_T = 1 << 3,
+ MEMPARSE_SUFFIX_P = 1 << 4,
+ MEMPARSE_SUFFIX_E = 1 << 5,
+};
+
+#define MEMPARSE_SUFFIXES_DEFAULT (MEMPARSE_SUFFIX_K | MEMPARSE_SUFFIX_M |\
+ MEMPARSE_SUFFIX_G | MEMPARSE_SUFFIX_T |\
+ MEMPARSE_SUFFIX_P)
+
+int __must_check memparse_safe(const char *s, enum memparse_suffix suffixes,
+ unsigned long long *res, char **retptr);
int __must_check kstrtoull(const char *s, unsigned int base, unsigned long long *res);
int __must_check kstrtoll(const char *s, unsigned int base, long long *res);

diff --git a/lib/cmdline.c b/lib/cmdline.c
index 90ed997d9570..d379157de349 100644
--- a/lib/cmdline.c
+++ b/lib/cmdline.c
@@ -139,12 +139,15 @@ char *get_options(const char *str, int nints, int *ints)
EXPORT_SYMBOL(get_options);

/**
- * memparse - parse a string with mem suffixes into a number
+ * memparse - DEPRECATED, parse a string with mem suffixes into a number
* @ptr: Where parse begins
* @retptr: (output) Optional pointer to next char after parse completes
*
+ * There is no way to handle errors, and no overflown detection and string
+ * sanity checks.
* Parses a string into a number. The number stored at @ptr is
* potentially suffixed with K, M, G, T, P, E.
+ *
*/

unsigned long long memparse(const char *ptr, char **retptr)
diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index d586e6af5e5a..38c772097301 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -113,6 +113,102 @@ static int _kstrtoull(const char *s, unsigned int base, unsigned long long *res)
return 0;
}

+/**
+ * memparse_safe - convert a string to an unsigned long long, safer version of
+ * memparse()
+ *
+ * @s: The start of the string. Must be null-terminated.
+ * The base would be determined automatically, if it starts with
+ * "0x" the base would be 16, if it starts with "0" the base
+ * would be 8, otherwise the base would be 10.
+ * After a valid number string, there can be at most one
+ * case-insensive suffix character, specified by the @suffixes
+ * parameter.
+ *
+ * @suffixes: The suffixes which should be parsed. Use logical ORed
+ * memparse_suffix enum to indicate the supported suffixes.
+ * The suffixes are case-insensive, all 2 ^ 10 based.
+ * Supported ones are "KMGPTE".
+ * NOTE: If one suffix out of the supported one is hit, it would
+ * end the parse normally, with @retptr pointed to the unsupported
+ * suffix.
+ *
+ * @res: Where to write the result.
+ *
+ * @retptr: (output) Optional pointer to the next char after parse completes.
+ *
+ * Return 0 if any valid numberic string can be parsed, and @retptr updated.
+ * Return -INVALID if no valid number string can be found.
+ * Return -ERANGE if the number overflows.
+ * For minus return values, @retptr would not be updated.
+ */
+noinline int memparse_safe(const char *s, enum memparse_suffix suffixes,
+ unsigned long long *res, char **retptr)
+{
+ unsigned long long value;
+ unsigned int rv;
+ int shift = 0;
+ int base = 0;
+
+ s = _parse_integer_fixup_radix(s, &base);
+ rv = _parse_integer(s, base, &value);
+ if (rv & KSTRTOX_OVERFLOW)
+ return -ERANGE;
+ if (rv == 0)
+ return -EINVAL;
+
+ s += rv;
+ switch (*s) {
+ case 'K':
+ case 'k':
+ if (!(suffixes & MEMPARSE_SUFFIX_K))
+ break;
+ shift = 10;
+ break;
+ case 'M':
+ case 'm':
+ if (!(suffixes & MEMPARSE_SUFFIX_M))
+ break;
+ shift = 20;
+ break;
+ case 'G':
+ case 'g':
+ if (!(suffixes & MEMPARSE_SUFFIX_G))
+ break;
+ shift = 30;
+ break;
+ case 'T':
+ case 't':
+ if (!(suffixes & MEMPARSE_SUFFIX_T))
+ break;
+ shift = 40;
+ break;
+ case 'P':
+ case 'p':
+ if (!(suffixes & MEMPARSE_SUFFIX_P))
+ break;
+ shift = 50;
+ break;
+ case 'E':
+ case 'e':
+ if (!(suffixes & MEMPARSE_SUFFIX_E))
+ break;
+ shift = 60;
+ break;
+ }
+ if (shift) {
+ s++;
+ if (value >> (64 - shift))
+ return -ERANGE;
+ value <<= shift;
+ }
+ *res = value;
+ if (retptr)
+ *retptr = (char *)s;
+ return 0;
+}
+EXPORT_SYMBOL(memparse_safe);
+
/**
* kstrtoull - convert a string to an unsigned long long
* @s: The start of the string. The string must be null-terminated, and may also
--
2.43.0