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Avoid int overflow after year 2038 #270

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Avoid int overflow after year 2038 #270

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2 changes: 1 addition & 1 deletion util.c
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Original file line number Diff line number Diff line change
Expand Up @@ -634,7 +634,7 @@ int check_ext2(int fd, char *name)
if (sb[56] != 0x53 || sb[57] != 0xef)
return 0;

mtime = sb[44]|(sb[45]|(sb[46]|sb[47]<<8)<<8)<<8;
mtime = sb[44]|(sb[45]|(sb[46]|(unsigned int)sb[47]<<8)<<8)<<8;
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@mwilck mwilck Jun 5, 2026 •
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This needs more explanation in the commit message. AFAIU this is effective because it overrides the standard integral promotion in C, which counter-intuitively promotes unsigned char to int if, as usual, the size of the latter type is larger than the size of the former.

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@mwilck mwilck Jun 5, 2026

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Extending this with explanation from claude:

  • sb is unsigned char[], so each element is unsigned char
  • C's integer promotion rules promote unsigned char to int before any arithmetic
  • All intermediate computations are therefore signed int
  • For timestamps after 2038-01-19 03:14:07 UTC, sb[47] >= 0x80 (the MSB has its high bit set)
  • Eventually this means a 1 is shifted into bit 31 of a signed int — this is undefined behavior in C (pre-C23), because left-shifting into the sign bit of a signed integer is UB
  • On every real implementation, this produces a negative int
  • When assigned to a 64-bit time_t (the type on all modern 64-bit Linux), the negative int is sign-extended to a negative time_t, representing a date before the Unix epoch — completely wrong

bsize = sb[24]|(sb[25]|(sb[26]|sb[27]<<8)<<8)<<8;
size = sb[4]|(sb[5]|(sb[6]|sb[7]<<8)<<8)<<8;
size <<= bsize;
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