path: root/lib
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authorLinus Torvalds <torvalds@linux-foundation.org>2015-10-04 16:31:13 +0100
committerLinus Torvalds <torvalds@linux-foundation.org>2015-10-04 16:31:13 +0100
commit30c44659f4a3e7e1f9f47e895591b4b40bf62671 (patch)
tree0bc2af55dd7f7e7fa0a2d1ff11b5929f5ed1fc9e /lib
parent15ecf9a986e2678f5de36ead23b89235612fc03f (diff)
parent30059d494a72603d066baf55c748803df968aa08 (diff)
Merge branch 'strscpy' of git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile
Pull strscpy string copy function implementation from Chris Metcalf. Chris sent this during the merge window, but I waffled back and forth on the pull request, which is why it's going in only now. The new "strscpy()" function is definitely easier to use and more secure than either strncpy() or strlcpy(), both of which are horrible nasty interfaces that have serious and irredeemable problems. strncpy() has a useless return value, and doesn't NUL-terminate an overlong result. To make matters worse, it pads a short result with zeroes, which is a performance disaster if you have big buffers. strlcpy(), by contrast, is a mis-designed "fix" for strlcpy(), lacking the insane NUL padding, but having a differently broken return value which returns the original length of the source string. Which means that it will read characters past the count from the source buffer, and you have to trust the source to be properly terminated. It also makes error handling fragile, since the test for overflow is unnecessarily subtle. strscpy() avoids both these problems, guaranteeing the NUL termination (but not excessive padding) if the destination size wasn't zero, and making the overflow condition very obvious by returning -E2BIG. It also doesn't read past the size of the source, and can thus be used for untrusted source data too. So why did I waffle about this for so long? Every time we introduce a new-and-improved interface, people start doing these interminable series of trivial conversion patches. And every time that happens, somebody does some silly mistake, and the conversion patch to the improved interface actually makes things worse. Because the patch is mindnumbing and trivial, nobody has the attention span to look at it carefully, and it's usually done over large swatches of source code which means that not every conversion gets tested. So I'm pulling the strscpy() support because it *is* a better interface. But I will refuse to pull mindless conversion patches. Use this in places where it makes sense, but don't do trivial patches to fix things that aren't actually known to be broken. * 'strscpy' of git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile: tile: use global strscpy() rather than private copy string: provide strscpy() Make asm/word-at-a-time.h available on all architectures
Diffstat (limited to 'lib')
1 files changed, 88 insertions, 0 deletions
diff --git a/lib/string.c b/lib/string.c
index 13d1e84ddb80..8dbb7b1eab50 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -27,6 +27,10 @@
#include <linux/bug.h>
#include <linux/errno.h>
+#include <asm/byteorder.h>
+#include <asm/word-at-a-time.h>
+#include <asm/page.h>
* strncasecmp - Case insensitive, length-limited string comparison
@@ -146,6 +150,90 @@ size_t strlcpy(char *dest, const char *src, size_t size)
+ * strscpy - Copy a C-string into a sized buffer
+ * @dest: Where to copy the string to
+ * @src: Where to copy the string from
+ * @count: Size of destination buffer
+ *
+ * Copy the string, or as much of it as fits, into the dest buffer.
+ * The routine returns the number of characters copied (not including
+ * the trailing NUL) or -E2BIG if the destination buffer wasn't big enough.
+ * The behavior is undefined if the string buffers overlap.
+ * The destination buffer is always NUL terminated, unless it's zero-sized.
+ *
+ * Preferred to strlcpy() since the API doesn't require reading memory
+ * from the src string beyond the specified "count" bytes, and since
+ * the return value is easier to error-check than strlcpy()'s.
+ * In addition, the implementation is robust to the string changing out
+ * from underneath it, unlike the current strlcpy() implementation.
+ *
+ * Preferred to strncpy() since it always returns a valid string, and
+ * doesn't unnecessarily force the tail of the destination buffer to be
+ * zeroed. If the zeroing is desired, it's likely cleaner to use strscpy()
+ * with an overflow test, then just memset() the tail of the dest buffer.
+ */
+ssize_t strscpy(char *dest, const char *src, size_t count)
+ const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
+ size_t max = count;
+ long res = 0;
+ if (count == 0)
+ return -E2BIG;
+ /*
+ * If src is unaligned, don't cross a page boundary,
+ * since we don't know if the next page is mapped.
+ */
+ if ((long)src & (sizeof(long) - 1)) {
+ size_t limit = PAGE_SIZE - ((long)src & (PAGE_SIZE - 1));
+ if (limit < max)
+ max = limit;
+ }
+ /* If src or dest is unaligned, don't do word-at-a-time. */
+ if (((long) dest | (long) src) & (sizeof(long) - 1))
+ max = 0;
+ while (max >= sizeof(unsigned long)) {
+ unsigned long c, data;
+ c = *(unsigned long *)(src+res);
+ *(unsigned long *)(dest+res) = c;
+ if (has_zero(c, &data, &constants)) {
+ data = prep_zero_mask(c, data, &constants);
+ data = create_zero_mask(data);
+ return res + find_zero(data);
+ }
+ res += sizeof(unsigned long);
+ count -= sizeof(unsigned long);
+ max -= sizeof(unsigned long);
+ }
+ while (count) {
+ char c;
+ c = src[res];
+ dest[res] = c;
+ if (!c)
+ return res;
+ res++;
+ count--;
+ }
+ /* Hit buffer length without finding a NUL; force NUL-termination. */
+ if (res)
+ dest[res-1] = '\0';
+ return -E2BIG;
* strcat - Append one %NUL-terminated string to another