From aafa5df0df39036c6500846acd3db5b75d264a3b Mon Sep 17 00:00:00 2001 From: Jonathan Nieder Date: Tue, 18 May 2021 18:52:56 -0700 Subject: [PATCH] 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 Signed-off-by: Junio C Hamano --- git-compat-util.h | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/git-compat-util.h b/git-compat-util.h index dcc786edaa..acef7f6bcb 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -981,11 +981,9 @@ static inline char *xstrdup_or_null(const char *str) static inline size_t xsize_t(off_t len) { - size_t size = (size_t) len; - - if (len != (off_t) size) + if (len < 0 || (uintmax_t) len > SIZE_MAX) die("Cannot handle files this big"); - return size; + return (size_t) len; } __attribute__((format (printf, 3, 4)))