Run images whose PT_LOAD covers the low 64 KiB #440

An ET_EXEC whose first PT_LOAD covers VDSO_BASE (0xF000) faults under elfuse. vdso_build writes the vDSO over the image's text at VDSO_BASE, and when the image starts at 0x0, the null page that guest_invalidate_ptes removes also takes its ELF header, so the first access to it faults.

Reproduction: aarch64-linux-gnu-gcc -static -O2 -Wl,-Ttext-segment=0x8000 hello.c (ld rounds the first PT_LOAD to 0x0), then elfuse ./a.out dies with SIGSEGV at 0x40. Linux 6.18 runs the same binary as root, where vm.mmap_min_addr does not apply.

The cause is the fixed VDSO_BASE. Linux places the vDSO in the mmap area, after the image is mapped, so it never collides with a PT_LOAD. Moving it in elfuse touches the shim, the signal trampoline address (VDSO_BASE + VDSO_OFF_SIGRET) and the boot and exec page-table setup. A smaller first step would refuse such an image at load with a clear error instead of faulting after the vDSO has been written over it.

Found while reviewing #435.