From fbbc51aa3edf1475ea4be0ca2639bd37ba897f4f Mon Sep 17 00:00:00 2001 From: Matteo Croce Date: Fri, 12 Dec 2025 19:28:54 -0800 Subject: [PATCH] cat: adjust the maximum data copied by copy_file_range copy_cat() is doing unnecessary math to derive the maximum size allowed by the copy_file_range() syscall. This calculates an excessively big number which is 0x7FFFFFFFC0000000. Linux syscalls have a much lower limit for single I/O operations, which is defined in kernel as `INT_MAX - PAGE_SIZE`, so lower the limit to that. evidence of the limit is shown below: $ uname -a Linux v 6.17.5 #191 SMP Thu Oct 23 21:43:33 CEST 2025 x86_64 GNU/Linux $ ll random -rw-r--r-- 1 teknoraver users 8.0G Dec 12 19:19 random $ strace -e copy_file_range src/cat random >random2 copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 2147479552 copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 2147479552 copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 2147479552 copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 2147479552 copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 16384 copy_file_range(3, NULL, 1, NULL, 9223372035781033984, 0) = 0 --- src/cat.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/src/cat.c b/src/cat.c index c02210301..811955459 100644 --- a/src/cat.c +++ b/src/cat.c @@ -503,10 +503,10 @@ cat (char *inbuf, idx_t insize, char *outbuf, idx_t outsize, static int copy_cat (void) { - /* Copy at most COPY_MAX bytes at a time; this is min - (SSIZE_MAX, SIZE_MAX) truncated to a value that is - surely aligned well. */ - ssize_t copy_max = MIN (SSIZE_MAX, SIZE_MAX) >> 30 << 30; + /* According to the read(2) man page, on Linux syscalls will transfer at most + * 0x7ffff000 (2,147,479,552) bytes on both 32-bit and 64-bit systems. + * In kernel this is defined as MAX_RW_COUNT which is INT_MAX - PAGE_SIZE. */ + ssize_t copy_max = 0x7ffff000; /* copy_file_range does not support some cases, and it incorrectly returns 0 when reading from the proc file -- 2.50.1