cloudbleed

Repair this code

Return how many bytes a run of reads gives back when the buffer only holds so much.

today's brief

February 17, 2017. A Google researcher fuzzing search results starts finding private chat messages and live session cookies sitting inside other people's web pages. Cloudflare's HTML rewriter had been reading past the end of its own buffer, shipping whatever sat next in memory straight out to browsers. Search engines cached the leaks for months, so the cleanup meant asking Google to purge its index. Roughly 150 customers were hit. Make the reads stay inside the buffer.

See it in action

emit({2}, {4}, 1, 16)

Expected result

Your checklist

For your own tracking. Tests grade your code.

The buffer holds len bytes, numbered 0 to len - 1.
Read i starts at offset[i] and asks for want[i] bytes. Both are at least 0.
A read gives back only the bytes that fall inside the buffer, so a read running off the end gives back the part that fits, and a read starting at or past the end gives back nothing.
Return the total number of bytes the n reads give back.

More examples

emit({12}, {10}, 1, 16)

Expected result

emit({20}, {5}, 1, 16)

Expected result

Constraints

Expected files

emit.c

Allowed functions

None. Write every helper yourself.

Back to compete

Solutions

cloudbleed

Repair this code

Return how many bytes a run of reads gives back when the buffer only holds so much.

today's brief

See it in action

emit({2}, {4}, 1, 16)

Expected result

Your checklist

For your own tracking. Tests grade your code.

The buffer holds len bytes, numbered 0 to len - 1.
Read i starts at offset[i] and asks for want[i] bytes. Both are at least 0.
A read gives back only the bytes that fall inside the buffer, so a read running off the end gives back the part that fits, and a read starting at or past the end gives back nothing.
Return the total number of bytes the n reads give back.

More examples

emit({12}, {10}, 1, 16)

Expected result

emit({20}, {5}, 1, 16)

Expected result

Constraints

Expected files

emit.c

Allowed functions

None. Write every helper yourself.

Back to compete

Solutions