[libc][string] fix strncpy potential buffer overflow #389
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The wrong placement of the increment for index `i` causes an unexpected behavior, which the `strncpy` writes an extra '\0'. For example: The `src` string is "abc". The buffer size of `dest` is 5. When we call `strncpy(dest, src, 5)`, the first `for` loop copies the characters, 'a', 'b', and 'c', to the `dest[0:2]`. In the 4th iteration, however, the `for` loop breaks due to the termination of `src` whereas the value of `i` stays 3. At the moment, it has copied 4 bytes, including the '\0' of `src`. In the second `for` loop, we have `i = 3` and `count = 5`, so the loop copies two more '\0' to the `dest`. As a result, the `strncpy` copies 6 bytes to the `dest` buffer, leading to buffer overflow. Fix the issue by increasing the index `i` before every copy. Signed-off-by: Cody Wong <[email protected]>
travisg
approved these changes
Nov 4, 2023
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The wrong placement of the increment for index
icauses an unexpected behavior, which thestrncpywrites an extra '\0'.For example:
The
srcstring is "abc". The buffer size ofdestis 5.When we call
strncpy(dest, src, 5), the firstforloop copies the characters, 'a', 'b', and 'c', to thedest[0:2]. In the 4th iteration, however, theforloop breaks due to the termination ofsrcwhereas the value ofistays 3. At the moment, it has copied 4 bytes, including the '\0' ofsrc.In the second
forloop, we havei = 3andcount = 5, so the loop copies two more '\0' to thedest. As a result, thestrncpycopies 6 bytes to thedestbuffer, leading to buffer overflow.Fix the issue by increasing the index
ibefore every copy.