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Ole Tange
Asked: 2024-12-30 03:53:08 +0800 CST

为什么我不能拦截`seq`中的`write`？

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根据限制文件 IO 速度，我构建了https://git.data.coop/tange/tangetools/src/branch/master/iothrottle

它适用于某些程序：

iothrottle -i 10M cat foo > bar # Yes, here you could just use pv
iothrottle -o 1M ffmpeg -i foo.mp3 foo.wav
iothrottle -o 1M cp -a foodir /other/fs/foodir

这不会：

iothrottle -o 1M cp -a foodir /same/fs/foodir

因为在同一个文件系统上每个文件cp使用一次调用copy_file_range。

我可以接受这个限制。

它对 也不起作用seq。当我运行时，strace seq 100000我得到：

write(1, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n14"..., 8192) = 8192
write(1, "\n1861\n1862\n1863\n1864\n1865\n1866\n1"..., 4096) = 4096
write(1, "2680\n2681\n2682\n2683\n2684\n2685\n26"..., 4096) = 4096
write(1, "499\n3500\n3501\n3502\n3503\n3504\n350"..., 4096) = 4096
write(1, "18\n4319\n4320\n4321\n4322\n4323\n4324"..., 4096) = 4096
write(1, "7\n5138\n5139\n5140\n5141\n5142\n5143\n"..., 4096) = 4096
write(1, "\n5957\n5958\n5959\n5960\n5961\n5962\n5"..., 4096) = 4096
write(1, "6776\n6777\n6778\n6779\n6780\n6781\n67"..., 4096) = 4096
write(1, "595\n7596\n7597\n7598\n7599\n7600\n760"..., 4096) = 4096
write(1, "14\n8415\n8416\n8417\n8418\n8419\n8420"..., 4096) = 4096
write(1, "3\n9234\n9235\n9236\n9237\n9238\n9239\n"..., 3838) = 3838

因此它似乎在调用write（而不是某些mmap或其他魔法），因此应该起作用。

但是当我运行时：IOTHROTTLE_DEBUG=1 iothrottle -o 1M seq 10000 >/dev/null我得到：

init called: default read_limit=0, write_limit=0
IOTHROTTLE_READ=0
IOTHROTTLE_WRITE=1048576
Final read_limit=0, write_limit=1048576
allowed: 608 written: 0
allowed: 617 written: 0

这告诉我iothrottle仅拦截了 2 次调用write。

我错过了什么？

shared-library

1 个回答

  1. Best Answer

    您iothrottle可以通过劫持 C 库的函数来完成工作，这些函数最终会执行诸如write()、pwrite()或splice()² 之类的系统调用。

    strace跟踪系统调用，而不是动态库函数调用，ltrace在 GNU 系统上您需要动态库函数调用（ltrace也可以使用 跟踪系统调用-S；在其他系统上，等效命令如truss或tusc（HP/UX）也可以同时执行这两项操作）。

    以下是来自zshDebian 的内容：

    $ ltrace seq 10 |& grep -e{write,put,send}
fwrite_unlocked("1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", 21, 1, 0x7f5b1d7185c0) = 1

    $ ltrace -S seq 10 |& grep -e{write,put,send}
fwrite_unlocked("1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", 21, 1, 0x7fe86c4785c0 <unfinished ...>
<... fwrite_unlocked resumed> )                  = 1
write@SYS(1, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n", 211

    seq正在调用fwrite_unlocked()您未劫持的 stdio 函数，fwrite()这不能解释为什么您的iothrottle操作对其无效。

    $ nm -uD  =seq | grep -e{write,put,send}
                 U fputc_unlocked@GLIBC_2.2.5
                 U fputs_unlocked@GLIBC_2.2.5
                 U fwrite@GLIBC_2.2.5
                 U fwrite_unlocked@GLIBC_2.2.5

    显示一些需要的-u未定义的-D动态符号seq（=seq可以在其他 shell 中写入"$(command -v seq)"）。

    在动态符号的环境中调用seq，LD_DEBUG=bindings显示关于在哪里为谁找到什么符号的信息，这也可以有所帮助：

    $ LD_PRELOAD=$PWD/iothrottle.so IOTHROTTLE_WRITE=1000 LD_DEBUG=bindings seq 10  |& grep -e{write,put,send,iothrot}
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `init'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_pread'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_write'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_pwrite'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_copy_file_range'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_fwrite'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_writev'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_fflush'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_readv'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_sendfile'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_splice'
     14982: binding file .../iothrottle.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `__cxa_finalize' [GLIBC_2.2.5]
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `original_read'
     14982: binding file .../iothrottle.so [0] to seq [0]: normal symbol `stderr' [GLIBC_2.2.5]
     14982: calling init: .../iothrottle.so
     14982: binding file .../iothrottle.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `clock_gettime' [GLIBC_2.17]
     14982: binding file .../iothrottle.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `getenv' [GLIBC_2.2.5]
     14982: binding file .../iothrottle.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `strtoull' [GLIBC_2.2.5]
     14982: binding file seq [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `fwrite_unlocked' [GLIBC_2.2.5]
     14982: binding file seq [0] to .../iothrottle.so [0]: normal symbol `fflush' [GLIBC_2.2.5]
     14982: binding file .../iothrottle.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `dlsym' [GLIBC_2.34]
     14982: binding file .../iothrottle.so [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `fflush'
     14982: binding file .../iothrottle.so [0] to .../iothrottle.so [0]: normal symbol `throttle'

    ¹ 或者可能最终像 stdio 那样，通常只填满一些缓冲区而不执行任何 I/O，直到这些缓冲区被刷新。

    ² 顺便说一句，还有更多诸如send()、、之类的函数也可以执行一些 I/O 操作sendto()，并且sendmsg()还有更多 stdio 函数可以执行一些操作，write()例如fput()、、......putchar()printf()

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