Report guest path for /proc/self/exe under Rosetta

[jserv]

On x86_64 guests, readlink("/proc/self/exe") and
readlink("/proc/self/fd/N") (with N from opening /proc/self/exe) returned the Rosetta translator path (ROSETTA_PATH) instead of the running guest binary. GNU coreutils ships a multi-call binary whose applets self-identify by reading /proc/self/exe, opening it, then readlink-ing /proc/self/fd/N and matching the basename. Getting "rosetta" back made every applet abort with "unknown program 'rosetta'", which broke Debian/Ubuntu maintainer scripts under apt upgrade and dpkg -i.

Factor the /proc/self/exe resolution (ELF-path snapshot plus sysroot-prefix strip) into proc_readlink_self_exe. Drop the Rosetta early-return from the direct /proc/self/exe branch, and in the /proc/self/fd/N branch substitute the guest binary when the host fd resolves to ROSETTA_PATH.

proc_intercept_open("/proc/self/exe") still hands back an fd to ROSETTA_PATH so the VZ ioctl gate (rosetta_ioctl_target_fd) keeps recognising Rosetta's own self-identification; only the guest-visible readlink string changed.

Close #107

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Summary by cubic

Fixes wrong /proc/self/exe reporting under Rosetta. readlink("/proc/self/exe") and readlink("/proc/self/fd/N") now return the guest binary path (sysroot-stripped) so multi-call binaries no longer abort.

• Bug Fixes

  • Return the guest binary for readlink("/proc/self/exe") and readlink("/proc/self/fd/N") on x86_64 under Rosetta, with sysroot prefix stripped.
  • Keep open("/proc/self/exe") returning a host fd to ROSETTA_PATH so the VZ ioctl gate still recognizes Rosetta.
  • Add a regression test asserting /proc/self/exe resolves to the guest (basename busybox), not Rosetta.

• Refactors

  • Extract proc_readlink_self_exe() to resolve the ELF path and strip the sysroot; reuse in both readlink paths.
  • Remove the Rosetta early-return for /proc/self/exe and substitute the guest path when an fd resolves to ROSETTA_PATH.

^{Written for commit ebd6d26. Summary will update on new commits.}