#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/uaccess.h>

#include <asm/word-at-a-time.h>

/* Set bits in the first 'n' bytes when loaded from memory */
#ifdef __LITTLE_ENDIAN
#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
#else
#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
#endif

/*
 * Do a strnlen, return length of string *with* final '\0'.
 * 'count' is the user-supplied count, while 'max' is the
 * address space maximum.
 *
 * Return 0 for exceptions (which includes hitting the address
 * space maximum), or 'count+1' if hitting the user-supplied
 * maximum count.
 *
 * NOTE! We can sometimes overshoot the user-supplied maximum
 * if it fits in a aligned 'long'. The caller needs to check
 * the return value against "> max".
 */
static inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
{
	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
	long align, res = 0;
	unsigned long c;

	/*
	 * Truncate 'max' to the user-specified limit, so that
	 * we only have one limit we need to check in the loop
	 */
	if (max > count)
		max = count;

	/*
	 * Do everything aligned. But that means that we
	 * need to also expand the maximum..
	 */
	align = (sizeof(long) - 1) & (unsigned long)src;
	src -= align;
	max += align;

	unsafe_get_user(c, (unsigned long __user *)src, efault);
	c |= aligned_byte_mask(align);

	for (;;) {
		unsigned long data;
		if (has_zero(c, &data, &constants)) {
			data = prep_zero_mask(c, data, &constants);
			data = create_zero_mask(data);
			return res + find_zero(data) + 1 - align;
		}
		res += sizeof(unsigned long);
		/* We already handled 'unsigned long' bytes. Did we do it all ? */
		if (unlikely(max <= sizeof(unsigned long)))
			break;
		max -= sizeof(unsigned long);
		unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
	}
	res -= align;

	/*
	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
	 * too? If so, return the marker for "too long".
	 */
	if (res >= count)
		return count+1;

	/*
	 * Nope: we hit the address space limit, and we still had more
	 * characters the caller would have wanted. That's 0.
	 */
efault:
	return 0;
}

/**
 * strnlen_user: - Get the size of a user string INCLUDING final NUL.
 * @str: The string to measure.
 * @count: Maximum count (including NUL character)
 *
 * Context: User context only. This function may sleep if pagefaults are
 *          enabled.
 *
 * Get the size of a NUL-terminated string in user space.
 *
 * Returns the size of the string INCLUDING the terminating NUL.
 * If the string is too long, returns a number larger than @count. User
 * has to check the return value against "> count".
 * On exception (or invalid count), returns 0.
 *
 * NOTE! You should basically never use this function. There is
 * almost never any valid case for using the length of a user space
 * string, since the string can be changed at any time by other
 * threads. Use "strncpy_from_user()" instead to get a stable copy
 * of the string.
 */
long strnlen_user(const char __user *str, long count)
{
	unsigned long max_addr, src_addr;

	if (unlikely(count <= 0))
		return 0;

	max_addr = user_addr_max();
	src_addr = (unsigned long)str;
	if (likely(src_addr < max_addr)) {
		unsigned long max = max_addr - src_addr;
		long retval;

		user_access_begin();
		retval = do_strnlen_user(str, count, max);
		user_access_end();
		return retval;
	}
	return 0;
}
EXPORT_SYMBOL(strnlen_user);

/**
 * strlen_user: - Get the size of a user string INCLUDING final NUL.
 * @str: The string to measure.
 *
 * Context: User context only. This function may sleep if pagefaults are
 *          enabled.
 *
 * Get the size of a NUL-terminated string in user space.
 *
 * Returns the size of the string INCLUDING the terminating NUL.
 * On exception, returns 0.
 *
 * If there is a limit on the length of a valid string, you may wish to
 * consider using strnlen_user() instead.
 */
long strlen_user(const char __user *str)
{
	unsigned long max_addr, src_addr;

	max_addr = user_addr_max();
	src_addr = (unsigned long)str;
	if (likely(src_addr < max_addr)) {
		unsigned long max = max_addr - src_addr;
		long retval;

		user_access_begin();
		retval = do_strnlen_user(str, ~0ul, max);
		user_access_end();
		return retval;
	}
	return 0;
}
EXPORT_SYMBOL(strlen_user);
on the linker to
associate each kcrctab slot with the correct value.

This has a couple of downsides:

 - Given that the CRCs are treated as memory addresses, we waste 4 bytes
   for each CRC on 64 bit architectures,

 - On architectures that support runtime relocation, a R_&lt;arch&gt;_RELATIVE
   relocation entry is emitted for each CRC value, which identifies it
   as a quantity that requires fixing up based on the actual runtime
   load offset of the kernel. This results in corrupted CRCs unless we
   explicitly undo the fixup (and this is currently being handled in the
   core module code)

 - Such runtime relocation entries take up 24 bytes of __init space
   each, resulting in a x8 overhead in [uncompressed] kernel size for
   CRCs.

Switching to explicit 32 bit values on 64 bit architectures fixes most
of these issues, given that 32 bit values are not treated as quantities
that require fixing up based on the actual runtime load offset.  Note
that on some ELF64 architectures [such as PPC64], these 32-bit values
are still emitted as [absolute] runtime relocatable quantities, even if
the value resolves to a build time constant.  Since relative relocations
are always resolved at build time, this patch enables MODULE_REL_CRCS on
powerpc when CONFIG_RELOCATABLE=y, which turns the absolute CRC
references into relative references into .rodata where the actual CRC
value is stored.

So redefine all CRC fields and variables as u32, and redefine the
__CRC_SYMBOL() macro for 64 bit builds to emit the CRC reference using
inline assembler (which is necessary since 64-bit C code cannot use
32-bit types to hold memory addresses, even if they are ultimately
resolved using values that do not exceed 0xffffffff).  To avoid
potential problems with legacy 32-bit architectures using legacy
toolchains, the equivalent C definition of the kcrctab entry is retained
for 32-bit architectures.

Note that this mostly reverts commit d4703aefdbc8 ("module: handle ppc64
relocating kcrctabs when CONFIG_RELOCATABLE=y")

Acked-by: Rusty Russell &lt;rusty@rustcorp.com.au&gt;
Signed-off-by: Ard Biesheuvel &lt;ard.biesheuvel@linaro.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;


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