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Fix potential overflow problem with timevals #646
When comparing a null timeval with a current timeval using curlx_tvdiff on a 32-bit linux system there's an overflow happening.
Compiling with -fsanitize=undefined I get this warning on startup of our app using libcurl:
.../curl/lib/timeval.c:120:37: runtime error: signed integer overflow: 5873565 * 1000 cannot be represented in type 'long int'
The attached fix solves the problem by checking for a null timeval before comparing. Strictly speaking the function probably should return something that's guaranteed to be 64-bit but the majority of the usecases actually cast the result to an int and basically just compares two timevals that are close together so I decided against making a bigger change.
Good catch! But since we use the *_tvdiff() function in quite a large number of places, maybe it would be better to fix that function to not do overflows? I'm thinking like this: