ee: Bound the number of bytes we'll skip in utf8_prev()

[markj]

Summary

utf8_prev() will walk backwards unless it sees a non-continuation byte,
but this is dangerous in the face of malformed UTF-8 input. In
particular, delete() will copy the skipped-over bytes into a
heap-allocated buffer, d_char, which is fixed at 5 bytes.

Make utf8_prev() refuse to step more than four bytes backward, to avoid
the overflow in delete().

Fixes: 62fba0054d9e ("ee: add unicode support")
Reported by: Sayono Hiragi

[markj]

Thinking a bit more, this is maybe not the right solution, delete() should perhaps handle invalid sequences instead.

[des]

contrib/ee/ee.c

245–247

de gustibus et coloribus non est disputandum, but I find this a little more readable.

[des]

In D57913#1327627, @markj wrote:

Thinking a bit more, this is maybe not the right solution, delete() should perhaps handle invalid sequences instead.

I was thinking along the same lines, because although utf8_prev() is now safe, it no longer does what it promises.

[des]

In D57913#1327630, @des wrote:

In D57913#1327627, @markj wrote:

Thinking a bit more, this is maybe not the right solution, delete() should perhaps handle invalid sequences instead.

I was thinking along the same lines, because although utf8_prev() is now safe, it no longer does what it promises.

d_char is allocated, so nothing prevents us from reallocating it (keeping track of its size of course) if we need to delete more than five bytes.