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forward_sendfile bypasses shadow_sp for writable shadow destination FDs #64

Description

@ChuWeiChang

Problem

When the guest application invokes sendfile(), forward_sendfile() in seccomp_dispatch.c incorrectly treats out_fd as physical host_fd when the destination out_fd is a shadowed file descriptor. It fails to check if the destination has an active shadow_sp.

Proposed Solution

Explicitly query the out_fd entry in the FD table before the write loop. If shadow_writeback is enabled and shadow_sp >= 0, extract the shadow_sp and route the sendfile writes to that virtual pointer instead of the raw host FD.

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