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| From the Linux kernel's point of view, applications differ by values set during exec of application such as <code>mmap_base</code> or thread info flags <code>TIF_ADDR32</code>/<code>TIF_IA32</code>/<code>TIF_X32</code>. | | From the Linux kernel's point of view, applications differ by values set during exec of application such as <code>mmap_base</code> or thread info flags <code>TIF_ADDR32</code>/<code>TIF_IA32</code>/<code>TIF_X32</code>. |
| Both native and compat applications can do 32 or 64-bit syscalls. | | Both native and compat applications can do 32 or 64-bit syscalls. |
| + | |
| + | == Mixed-bitness applications == |
| + | |
| + | That's entirely possible with current kernel ABI to create mixed-bitness applications, which may be ''very'' entangled. |
| + | For example, one could set ''both'' 32-bit and 64-bit robust futex list pointers. |
| + | Or one can create multi-threaded application where some threads are executing 32-bit code, some 64-bit code. |
| + | |
| + | If we ever meet application of such mixed-bitness kind, the support may be added to CRIU quite easily, but it should be done under some compile-time config as it'll add more syscalls to usual C/R where they aren't needed. |
| + | |
| + | At this moment there is no plans to add such support and it's quite unlikely that we'll find such application in real world (non-syntetic test). |
| | | |
| == Approaches to C/R compatible applications == | | == Approaches to C/R compatible applications == |
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| | | |
| == Needs to be done (TODO) == | | == Needs to be done (TODO) == |
− |
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− | === Bug with mmaping over 4Gb ===
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− |
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− | As 32-bit application is restored from 64-bit CRIU, some task's properties that were filled on <code>exec()</code> are left, which is quite unusual for 32-bit task. One of the things, left from 64-bit binary is precalculated <code>mmap_base</code> which is used to find task's top/bottom address limit during <code>mmap()</code> syscall. That means that compat <code>sys_mmap()</code> may map page over 4Gb address and return 4-byte pointer to low bytes of address. Looks like no one has used compatible mmap in 64-bit binary. Results in broken mmap in restored 32-bit application, which can map vma over 4Gb.
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− |
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− | Patches to fix this bug at this moment were posted on lkml, but not yet accepted. See [[Upstream kernel commits]]. If they will not go to v4.9-stable, the kerndat test for 32-bit C/R will be reworked to check if the bug present in kernel (which is not nice thing, but ok).
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− |
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− | === List of failed tests ===
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− |
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− | The table is being kept up-to-date by [[User:Dsafonov|Dsafonov]] with latest kernel/CRIU patches in his environment, some of which may be yet not in tree or even yet not sent.
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− |
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− | {| class="wikitable"
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− | ! Name
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− | ! Fail reason
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− | |-
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− | | futex-rl || sys_get_robust_list() should be compat syscall for 32-bit tasks: kernel keeps two different lists: <code>robust_list</code> and <code>compat_robust_list</code> in <code>task_struct</code>
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− | |-
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− | | autofs || test's bug - hangs without C/R
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− | |-
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− | | vdso01 || need to add 32-bit vDSO symbols: __kernel_* (and check them with handlers in test)
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− | |-
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− | | vdso01 || fails at fault-inject trampoline testing: correct jump trampolines
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− | |}
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− |
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− | === Fixes for older kernels ===
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− |
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− | For kernels with backported mainline patches for 32-bit C/R (like vzkernel) there are a couple of things to do like different sizes of vdso/vvar (or vvar may not be even present).
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| | | |
| === Kernel patch for vsyscall page === | | === Kernel patch for vsyscall page === |
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| At this moment we'll support only compat ia32 applications, attempt to dump x32 compat binary should result in error. | | At this moment we'll support only compat ia32 applications, attempt to dump x32 compat binary should result in error. |
| + | |
| + | === Continue removing TIF_IA32 from uprobes & Oprofile === |
| + | |
| + | This flag should be gone as it's suggested by Andy & Oleg. |
| + | There is quite lot of work to make kernel work without it, but small gain: |
| + | the restored ia32 process will be traced by uprobes/oprofile and stuff like that. |
| + | |
| + | '''Updated''': Done by [https://lore.kernel.org/all/20201004032536.1229030-1-krisman@collabora.com/T/#u patches] - that merged to v5.11 |
| | | |
| == External links == | | == External links == |
− | * [https://github.com/xemul/criu/issues/43 github issue] | + | * [https://github.com/checkpoint-restore/criu/issues/43 github issue] |
| | | |
− | [[Category: Development]]
| |
| [[Category: Under the hood]] | | [[Category: Under the hood]] |