% cat pread.c #define _GNU_SOURCE #include <assert.h> #include <errno.h> #include <fcntl.h> #include <stdio.h> #include <string.h> #include <unistd.h> #include <sys/mman.h> void pread_v(int fd, void* buf, size_t nbytes, off64_t offset) { printf("pread(_, _, 0x%lx, _) -> ", nbytes); int ret = pread(fd, buf, nbytes, offset); if (ret != -1) { printf("0x%x\n", ret); } else { printf("%s\n", strerror(errno)); } } int main() { int mem = open("/proc/self/mem", O_RDONLY); assert(mem != -1); char buf[0x4000]; pread_v(mem, buf, 0x1000, 0); // should be EIO printf("---\n"); void* p1 = mmap(NULL, 0x2000, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0); void* p2 = mmap(p1 + 0x1000, 0x1000, PROT_NONE, MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0); assert(p1 != MAP_FAILED); assert(p2 == p1 + 0x1000); assert(munmap(p2, 0x1000) == 0); for (int n = sizeof(buf); n >= 0x1000; n /= 2) { pread_v(mem, buf, n, (off64_t)p1); // should return 0x1000 } return 0; } % /compat/linux/bin/gcc -std=c99 -Wall pread.c -o pread % ./pread pread(_, _, 0x1000, _) -> Bad address --- pread(_, _, 0x4000, _) -> Bad address pread(_, _, 0x2000, _) -> Bad address pread(_, _, 0x1000, _) -> 0x1000 On the other hand, Linux (Xubuntu 18.04.3) prints Input/Output errors and 0x1000, 0x1000, 0x1000 there.
(This may or may not be relevant to the Valve Anti-Cheat functionality in Steam.)
s/errors/error/
https://reviews.freebsd.org/D26816
A commit references this bug: Author: trasz Date: Tue Oct 20 17:24:30 UTC 2020 New revision: 366900 URL: https://svnweb.freebsd.org/changeset/base/366900 Log: Fix linprocfs(4) /proc/self/mem semantics to more closely match Linux. Steam's Anti-Cheat might depend on it. PR: 248223 Analyzed by: Alex S <iwtcex@gmail.com> Reviewed by: kib MFC after: 2 weeks Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D26816 Changes: head/sys/compat/linprocfs/linprocfs.c
Already there in 13.