bspatch: add integer overflow checks

[emaste]

Summary

Introduce a new add_off_t static function that exits with an error message if there's an overflow, otherwise returns their sum. Use this when adding values obtained from the input patch.

[kib]

usr.bin/bsdiff/bspatch/bspatch.c

78

Is this variant of the code so much better than explicit overflow checks below that it warrants coding the same check twice ? You need to do the explicit check anyway, so why bother ?

What can be done, instead, is to add e.g. sys/cdefs.h macro ADD_OF_CHECKED(a,b,result) and make this functionality generally useful, to avoid open-coding the same fragment several times. typeof() macro or generic C11 facility would allow to make that type-neutral.

[kib]

usr.bin/bsdiff/bspatch/bspatch.c

78

Or may be sys/param.h is better place.

[emaste]

usr.bin/bsdiff/bspatch/bspatch.c

78

We could do away with the bzctrllen > OFF_MAX - HEADER_SIZE, bzctrllen + HEADER_SIZE > OFF_MAX - bzdatalen and bzctrllen + HEADER_SIZE > OFF_MAX - bzdatalen checks if we were so inclined.

This approach is harder to get wrong (by missing an explicit overflow check), and is resilient to future changes in variable types.

[delphij]

Looks good to me in principal.

usr.bin/bsdiff/bspatch/bspatch.c

70

Maybe inline?

[emaste]

rebase, add inline as suggested by @delphij