1bd4403d86
When I initially added the unsafe_[get|put]_user() helpers in commit
5b24a7a2aa ("Add 'unsafe' user access functions for batched
accesses"), I made the mistake of modeling the interface on our
traditional __[get|put]_user() functions, which return zero on success,
or -EFAULT on failure.
That interface is fairly easy to use, but it's actually fairly nasty for
good code generation, since it essentially forces the caller to check
the error value for each access.
In particular, since the error handling is already internally
implemented with an exception handler, and we already use "asm goto" for
various other things, we could fairly easily make the error cases just
jump directly to an error label instead, and avoid the need for explicit
checking after each operation.
So switch the interface to pass in an error label, rather than checking
the error value in the caller. Best do it now before we start growing
more users (the signal handling code in particular would be a good place
to use the new interface).
So rather than
if (unsafe_get_user(x, ptr))
... handle error ..
the interface is now
unsafe_get_user(x, ptr, label);
where an error during the user mode fetch will now just cause a jump to
'label' in the caller.
Right now the actual _implementation_ of this all still ends up being a
"if (err) goto label", and does not take advantage of any exception
label tricks, but for "unsafe_put_user()" in particular it should be
fairly straightforward to convert to using the exception table model.
Note that "unsafe_get_user()" is much harder to convert to a clever
exception table model, because current versions of gcc do not allow the
use of "asm goto" (for the exception) with output values (for the actual
value to be fetched). But that is hopefully not a limitation in the
long term.
[ Also note that it might be a good idea to switch unsafe_get_user() to
actually _return_ the value it fetches from user space, but this
commit only changes the error handling semantics ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
122 lines
3 KiB
C
122 lines
3 KiB
C
#include <linux/compiler.h>
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#include <linux/export.h>
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#include <linux/kasan-checks.h>
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#include <linux/uaccess.h>
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#include <linux/kernel.h>
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#include <linux/errno.h>
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#include <asm/byteorder.h>
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#include <asm/word-at-a-time.h>
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#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
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#define IS_UNALIGNED(src, dst) 0
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#else
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#define IS_UNALIGNED(src, dst) \
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(((long) dst | (long) src) & (sizeof(long) - 1))
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#endif
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/*
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* Do a strncpy, return length of string without final '\0'.
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* 'count' is the user-supplied count (return 'count' if we
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* hit it), 'max' is the address space maximum (and we return
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* -EFAULT if we hit it).
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*/
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static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
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{
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const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
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long res = 0;
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/*
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* Truncate 'max' to the user-specified limit, so that
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* we only have one limit we need to check in the loop
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*/
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if (max > count)
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max = count;
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if (IS_UNALIGNED(src, dst))
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goto byte_at_a_time;
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while (max >= sizeof(unsigned long)) {
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unsigned long c, data;
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/* Fall back to byte-at-a-time if we get a page fault */
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unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
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*(unsigned long *)(dst+res) = c;
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if (has_zero(c, &data, &constants)) {
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data = prep_zero_mask(c, data, &constants);
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data = create_zero_mask(data);
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return res + find_zero(data);
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}
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res += sizeof(unsigned long);
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max -= sizeof(unsigned long);
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}
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byte_at_a_time:
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while (max) {
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char c;
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unsafe_get_user(c,src+res, efault);
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dst[res] = c;
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if (!c)
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return res;
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res++;
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max--;
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}
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/*
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* Uhhuh. We hit 'max'. But was that the user-specified maximum
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* too? If so, that's ok - we got as much as the user asked for.
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*/
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if (res >= count)
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return res;
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/*
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* Nope: we hit the address space limit, and we still had more
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* characters the caller would have wanted. That's an EFAULT.
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*/
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efault:
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return -EFAULT;
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}
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/**
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* strncpy_from_user: - Copy a NUL terminated string from userspace.
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* @dst: Destination address, in kernel space. This buffer must be at
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* least @count bytes long.
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* @src: Source address, in user space.
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* @count: Maximum number of bytes to copy, including the trailing NUL.
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*
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* Copies a NUL-terminated string from userspace to kernel space.
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*
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* On success, returns the length of the string (not including the trailing
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* NUL).
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*
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* If access to userspace fails, returns -EFAULT (some data may have been
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* copied).
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*
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* If @count is smaller than the length of the string, copies @count bytes
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* and returns @count.
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*/
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long strncpy_from_user(char *dst, const char __user *src, long count)
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{
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unsigned long max_addr, src_addr;
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if (unlikely(count <= 0))
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return 0;
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max_addr = user_addr_max();
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src_addr = (unsigned long)src;
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if (likely(src_addr < max_addr)) {
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unsigned long max = max_addr - src_addr;
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long retval;
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kasan_check_write(dst, count);
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user_access_begin();
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retval = do_strncpy_from_user(dst, src, count, max);
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user_access_end();
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return retval;
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}
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return -EFAULT;
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}
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EXPORT_SYMBOL(strncpy_from_user);
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