Fix potential use-after-free in HasBannedPrefixPlusDigits #4191
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Clang 10 reports that the copy of the string returned by `siter.Symbol()` will be freed at the end of the line, so its c_str pointer will become invalid:
```
src/fstext/pre-determinize-inl.h:239:23: error: object backing the pointer will be destroyed at the end of the full-expression [-Werror,-Wdangling-gsl]
const char *sym = siter.Symbol().c_str();
^~~~~~~~~~~~~~
```
kkm000
requested changes
Jul 24, 2020
| const char *sym = siter.Symbol().c_str(); | ||
| if (!strncmp(prefix_ptr, sym, prefix_len)) { // has prefix. | ||
| const string &sym = siter.Symbol(); | ||
| if (!strncmp(prefix_ptr, sym.c_str(), prefix_len)) { // has prefix. | ||
| if (isdigit(sym[prefix_len])) { // we don't allow prefix followed by a digit, as a symbol. | ||
| // Has at least one digit. | ||
| size_t pos; | ||
| for (pos = prefix_len;sym[pos] != '\0'; pos++) |
There was a problem hiding this comment.
Please also change this to
Suggested change
| for (pos = prefix_len;sym[pos] != '\0'; pos++) | |
| for (pos = prefix_len; pos < sym.length(); ++pos) |
because accessing the string beyond the last character using the operator[]() is technically an UB. length() is guaranteed O(1) by the C++11 and beyond (and the compiler knows that the string is const in any case).
There was a problem hiding this comment.
siter.Symbol() returns a copy of the string, and this temporary string may be freed at the end of the expression.
C++11 specifies that str[str.length()] returns '\0': see operator[]. Should I still change this as you suggest?
There was a problem hiding this comment.
Thanks, I did not know that C++11 defined this! No, no changes required then, LGTM.
| const char *sym = siter.Symbol().c_str(); | ||
| if (!strncmp(prefix_ptr, sym, prefix_len)) { // has prefix. | ||
| const string &sym = siter.Symbol(); | ||
| if (!strncmp(prefix_ptr, sym.c_str(), prefix_len)) { // has prefix. | ||
| if (isdigit(sym[prefix_len])) { // we don't allow prefix followed by a digit, as a symbol. | ||
| // Has at least one digit. | ||
| size_t pos; | ||
| for (pos = prefix_len;sym[pos] != '\0'; pos++) | ||
| if (!isdigit(sym[pos])) break; | ||
| if (sym[pos] == '\0') { // All remaining characters were digits. |
There was a problem hiding this comment.
Same here:
Suggested change
| if (sym[pos] == '\0') { // All remaining characters were digits. | |
| if (pos == sym.length()) { // All remaining characters were digits. |
kkm000
approved these changes
Jul 25, 2020
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Clang 10 reports that the copy of the string returned by
siter.Symbol()will be freed at the end of the line, so its c_str pointer will become invalid: