Prefer WithReadonlyTempMount for diff reads#6787
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WriteUpperdir and winDiffer.Compare nest two WithTempMount calls. If the inner mount shares any backing directory with the outer one (e.g. the inner mount's lower is a subset of the outer's lowers), mounting the same upperdir read-write twice concurrently is undefined behavior on overlayfs. WithReadonlyTempMount strips upperdir/workdir from overlay mounts, so the kernel never sets up the writable side and the two mounts cannot collide. Also, mounting them read-only better expresses intent and prevents accidental writes from inside the callback. Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
tonistiigi
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May 20, 2026
| } | ||
| return mount.WithTempMount(ctx, lower, func(lowerRoot string) error { | ||
| return mount.WithTempMount(ctx, upperView, func(upperViewRoot string) error { | ||
| return mount.WithReadonlyTempMount(ctx, lower, func(lowerRoot string) error { |
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mounting the same upperdir read-write twice concurrently is undefined behavior on overlayfs.
But this mount only sets lowerdir, so there is no upperdir defined at all and already readonly.
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WriteUpperdir and winDiffer.Compare nest two WithTempMount calls. If the inner mount shares any backing directory with the outer one (e.g. the inner mount's lower is a subset of the outer's lowers), mounting the same upperdir read-write twice concurrently is undefined behavior on overlayfs.
WithReadonlyTempMount strips upperdir/workdir from overlay mounts, so the kernel never sets up the writable side and the two mounts cannot collide.
Also, mounting them read-only better expresses intent and prevents accidental writes from inside the callback.