[Bug string/19411] New: mbrtoc16 and mbrtowc: UB when n is larger than input size

["cherepan at mccme dot ru" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19411

            Bug ID: 19411
           Summary: mbrtoc16 and mbrtowc: UB when n is larger than input
                    size
           Product: glibc
           Version: 2.22
            Status: NEW
          Severity: normal
          Priority: P2
         Component: string
          Assignee: unassigned at sourceware dot org
          Reporter: cherepan at mccme dot ru
  Target Milestone: ---

The functions

size_t mbrtowc (wchar_t *pwc, const char *s, size_t n, mbstate_t *ps)
size_t mbrtoc16 (char16_t *pc16, const char *s, size_t n, mbstate_t *ps)

have a limit n on the number of processed bytes but it seems that the source
array s is not required to contain n accessible bytes. At least the commit

https://sourceware.org/git/?p=glibc.git;a=commit;h=16edf13b226dbe5f4f39794cfa363904af2c7973

added checks for pointer wrapping into mbrtowc and a test case with n=SIZE_MAX
(the test case uses mbtowc which in turn passes everything to __mbrtowc).

The check looks like this:

 endbuf = inbuf + n;
 if (__glibc_unlikely (endbuf < inbuf))

https://sourceware.org/git/?p=glibc.git;a=blob;f=wcsmbs/mbrtowc.c;h=dbfe9403a64a544a233a7b9d8f6bcab92d9edb28;hb=HEAD#l74
https://sourceware.org/git/?p=glibc.git;a=blob;f=wcsmbs/mbrtoc16.c;h=f23b24262b8fffaeed67c57155e47379e44b48fd;hb=HEAD#l92

This code is invalid. The sum "inbuf + n" is undefined by the C standard when
the result doesn't point into the same array. Then, checks for pointer wrapping
like "endbuf < inbuf" are already "miscompiled" by clang and, I guess, could be
expected to be broken by gcc in the future.

Similar to pr19391.

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