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authorLibravatar Jonathan Nieder <jrnieder@gmail.com>2021-05-18 18:52:56 -0700
committerLibravatar Junio C Hamano <gitster@pobox.com>2021-05-19 15:00:30 +0900
commitaafa5df0df39036c6500846acd3db5b75d264a3b (patch)
treec1ea39d1da88431ecdbb8522ade19fd6c5c19c58 /t/t3414-rebase-preserve-onto.sh
parentGit 2.31.1 (diff)
downloadtgif-aafa5df0df39036c6500846acd3db5b75d264a3b.tar.xz
xsize_t: avoid implementation defined behavior when len < 0
The xsize_t helper aims to safely convert an off_t to a size_t, erroring out when a file offset is too large to fit into a memory address. It does this by using two casts: size_t size = (size_t) len; if (len != (off_t) size) ... error out ... On a platform with sizeof(size_t) < sizeof(off_t), this check is safe and correct. The first cast truncates to a size_t by finding the remainder modulo SIZE_MAX+1 (see C99 section 6.3.1.3 Signed and unsigned integers) and the second promotes to an off_t, meaning the result is true if and only if len is representable as a size_t. On other platforms, this two-casts strategy still works well (always succeeds) for len >= 0. But for len < 0, when the first cast succeeds and produces SIZE_MAX + 1 + len, the resulting value is too large to be represented as an off_t, so the second cast produces implementation defined behavior. In practice, it is likely to produce a result of true despite len not being representable as size_t. Simplify by replacing with a more straightforward check: compare len to the relevant bounds and then cast it. (To avoid a -Wsign-compare warning, after checking that len >= 0, we explicitly convert to a sufficiently-large unsigned type before comparing to SIZE_MAX.) In practice, this is not likely to come up since typical callers use nonnegative len. Still, it's helpful to handle this case to make the behavior easy to reason about. Historical note: the original bounds-checking in 46be82dfd0 (xsize_t: check whether we lose bits, 2010-07-28) did not produce this implementation-defined behavior, though it still did not handle negative offsets. It was not until 73560c793a (git-compat-util.h: xsize_t() - avoid -Wsign-compare warnings, 2017-09-21) introduced the double cast that the implementation-defined behavior was triggered. Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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