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SHARD-794: a VM guest links /dev/fd and the stdio names - #632
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A vz or Firecracker guest boots a bare devtmpfs, which has no /dev/fd, /dev/stdin, /dev/stdout or /dev/stderr. runc, gVisor and Sysbox get them from the runtime, so bash <(...) and >(...) worked there and failed in a VM sandbox. bootGuest now links the four after devpts, and a conformance case holds every substrate to them.
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presmihaylov
commented
Oct 7, 2026
…lve, not their Linux targets
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SHARD-794 (task list outside the repo). Chief-approved overnight for Pres.
Why. A vz or Firecracker guest boots a bare devtmpfs, which has no
/dev/fd,/dev/stdin,/dev/stdoutor/dev/stderr. runc, gVisor and Sysbox get these links from the runtime, so bash<(...)and>(...)worked there and failed in a VM sandbox.How.
bootGuestincmd/shard-init/guest_linux.go: it links the four into the moved devtmpfs right after devpts. It runs once per boot, so the links never exist already.linkStdiolives inguest.goso its unit test runs on macOS.What. New conformance case
ExecFindsTheStdioLinksUnderDev.Tested e2e: vz (laptop, real VM: fails without the fix, passes with it), runc, gVisor, Sysbox (Lima). Firecracker shares
bootGuestbut has no KVM box.