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      LibcarePlus is a hot patch framework for user-mode processes. It can perform hot patch operations on target processes running on the Linux system without restarting the processes. Hot patches can be used to fix CVEs and urgent bugs that do not interrupt application services.

      Hardware and Software Requirements

      The following software and hardware requirements must be met to use LibcarePlus on openEuler:

      • Currently, the x86 and ARM64 architectures are supported.

      • LibcarePlus can run on any Linux distribution that supports libunwind, elfutils, and binutils.

      • LibcarePlus uses the ptrace() system call, which requires the kernel configuration option enabled for the corresponding Linux distribution.

      • LibcarePlus needs the symbol table of the original executable file when creating a hot patch. Do not strip the symbol table too early.

      • On the Linux OS where SELinux is enabled, manually adapt the SELinux policies.

      Precautions and Constraints

      When using LibcarePlus, comply with the following hot patch specifications and constraints:

      • Only the code written in the C language is supported. The assembly language is not supported.
      • Only user-mode programs are supported. Dynamic library patches are not supported.
      • The code file name must comply with the C language identifier naming specifications. That is, the code file name consists of letters (A-Z and a-z), digits (0-9), and underscores (_) but the first character cannot be a digit. Special characters such as hyphens (-) and dollar signs ($) are not allowed.
      • Incremental patches are supported. Multiple patches can be installed on a process. However, you need to design the patch installation and uninstallation management. Generally, the installation and uninstallation comply with the first-in, last-out (FILO) rule.
      • Automatic patch loading is not natively supported. You can design an automatic patch loading method for a specific process.
      • Patch query is supported.
      • The static function patch is restricted by the symbol table that can find the function in the system.
      • Hot patches are process-specific. That is, a hot patch of a dynamic library can be applied only to process that invoke the dynamic library.
      • The number of patches that can be applied to a process is limited by the range of the jump instruction and the size of the hole in the virtual memory address space. Generally, up to 512 patches can be applied to a process.
      • Thread local storage (TLS) variables of the initial executable (IE) model can be modified.
      • Symbols defined in a patch cannot be used in subsequent patches.
      • Hot patches are not supported in the following scenarios:
        • Infinite loop function, non-exit function, inline function, initialization function, and non-maskable interrupt (NMI) function
        • Replacing global variables
        • Functions less than 5 bytes
        • Modifying the header file
        • Adding or deleting the input and output parameters of the target function
        • Changing (adding, deleting, or modifying) data structure members
        • Modifying the C files that contain GCC macros such as LINE and FILE
        • Modifying the Intel vector assembly instruction

      Installing LibcarePlus

      Software Installation Dependencies

      The LibcarePlus running depends on libunwind, elfutils, and binutils. On the openEuler system configured with the Yum repo, you can run the following commands to install the software on which LibcarePlus depends:

      yum install -y binutils elfutils elfutils-libelf-devel libunwind-devel

      Installing LibcarePlus

      yum install libcareplus libcareplus-devel -y

      Check whether LibcarePlus is installed.

      # libcare-ctl -h
      usage: libcare-ctl [options] <cmd> [args]
        -v          - verbose mode
        -h          - this message
        patch  - apply patch to a user-space process
        unpatch- unapply patch from a user-space process
        info   - show info on applied patches

      Creating LibcarePlus Hot Patches


      LibcarePlus hot patch creation methods:

      • Manual creation
      • Creation through a script

      The process of manually creating a hot patch is complex. For a project with a large amount of code, for example, QEMU, it is extremely difficult to manually create a hot patch. You are advised to use the script provided by LibcarePlus to generate a hot patch file with one click.

      Manual Creation

      The following takes the original file foo.c and the patch file bar.c as examples to describe how to manually create a hot patch.

      1. Prepare the original file and patch file written in the C language. For example, foo.c and bar.c.

        Expand foo.c

        // foo.c
        #include <stdio.h>
        #include <time.h>
        void print_hello(void)
            printf("Hello world!\n");
        int main(void)
            while (1) {

        Expand bar.c

        // bar.c
        #include <stdio.h>
        #include <time.h>
        void print_hello(void)
            printf("Hello world %s!\n", "being patched");
        int main(void)
            while (1) {

      2. Build the original file and patch file to obtain the assembly files foo.s and bar.s.

        $ gcc -S foo.c
        $ gcc -S bar.c
        $ ls
        bar.c  bar.s  foo.c  foo.s
      3. Run kpatch_gensrc to compare foo.s and bar.s and generate the foobar.s file that contains the assembly content of the original file and the differences.

        sed -i 's/bar.c/foo.c/' bar.s
        kpatch_gensrc --os=rhel6 -i foo.s -i bar.s -o foobar.s --force-global

        By default, kpatch_gensrc compares the original files in the same C language. Therefore, before the comparison, you need to run the sed command to change the file name bar.c in the patch assembly file bar.s to the original file name foo.c. Call kpatch_gensrc to specify the input files as foo.s and bar.s and the output file as foobar.s.

      4. Build the assembly file foo.s in the original file and the generated assembly file foobar.s to obtain the executable files foo and foobar.

        gcc -o foo foo.s
        gcc -o foobar foobar.s -Wl,-q

        The -Wl, -q linker options reserve the relocation sections in foobar.

      5. Use kpatch_strip to remove the duplicate content from the executables foo and foobar and reserve the content required for creating hot patches.

        kpatch_strip --strip foobar foobar.stripped
        kpatch_strip --rel-fixup foo foobar.stripped
        strip --strip-unneeded foobar.stripped
        kpatch_strip --undo-link foo foobar.stripped

        The options in the preceding command are described as follows:

        • --strip removes useless sections for patch creation from foobar.
        • --rel-fixup repairs the address of the variables and functions accessed in the patch.
        • strip --strip-unneeded removes the useless symbol information for hot patch relocation.
        • --undo-link changes the symbol address in a patch from absolute to relative.
      6. Create a hot patch file.

        After the preceding operations, the contents required for creating the hot patch are obtained. Run the kpatch_make command to input parameters Build ID of the original executable file and foobar.stripped (output file of kpatch_strip) to kpatch_make to generate a hot patch file.

        $ str=$(readelf -n foo | grep 'Build ID')
        $ substr=${str##* }
        $ kpatch_make -b $substr -i 0001 foobar.stripped -o foo.kpatch
        $ ls
        bar.c  bar.s  foo  foobar  foobar.s  foobar.stripped  foo.c  foo.kpatch  foo.s

        The final hot patch file foo.kpatch whose patch ID is 0001 is obtained.

      Creation Through a Script

      This section describes how to use LibcarePlus built-in libcare-patch-make script to create a hot patch file. The original file foo.c and patch file bar.c are used as an example.

      1. Run the diff command to generate the comparison file of foo.c and bar.c.

        # diff -up foo.c bar.c > foo.patch

        The content of the foo.patch file is as follows:

        Expand foo.patch

        --- foo.c 2020-12-09 15:39:51.159632075 +0800
        +++ bar.c 2020-12-09 15:40:03.818632220 +0800
        @@ -1,10 +1,10 @@
        -// foo.c
        +// bar.c
        #include <stdio.h>
        #include <time.h>
        void print_hello(void)
        -    printf("Hello world!\n");
        +    printf("Hello world %s!\n", "being patched");
        int main(void)

      2. Write the makefile for building foo.c as follows:

        Expand makefile

        all: foo
        foo: foo.c
        $(CC) -o $@ $<
        rm -f foo
        install: foo
        mkdir $$DESTDIR || :
        cp foo $$DESTDIR

      3. After the makefile is done, directly call libcare-patch-make. If libcare-patch-make asks you which file to install the patch, enter the original file name, as shown in the following:

        # libcare-patch-make --clean -i 0001 foo.patch
        rm -f foo
        /usr/local/bin/libcare-cc -o foo foo.c
        INSTALLING ORIGINAL OBJECTS INTO /libcareplus/test/lpmake
        mkdir $DESTDIR || :
        cp foo $DESTDIR
        applying foo.patch...
        can't find file to patch at input line 3
        Perhaps you used the wrong -p or --strip option?
        The text leading up to this was:
        |--- foo.c 2020-12-10 09:43:04.445375845 +0800
        |+++ bar.c 2020-12-10 09:48:36.778379648 +0800
        File to patch: foo.c         
        patching file foo.c
        /usr/local/bin/libcare-cc -o foo foo.c
        INSTALLING PATCHED OBJECTS INTO /libcareplus/test/.lpmaketmp/patched
        mkdir $DESTDIR || :
        cp foo $DESTDIR
        Fixing up relocation printf@@GLIBC_2.2.5+fffffffffffffffc
        Fixing up relocation print_hello+0
        patch for /libcareplus/test/lpmake/foo is in /libcareplus/test/patchroot/700297b7bc56a11e1d5a6fb564c2a5bc5b282082.kpatch

        After the command is executed, the output indicates that the hot patch file is in the patchroot directory of the current directory, and the executable file is in the lpmake directory. By default, the Build ID is used to name a hot patch file generated by a script.

      Applying the LibcarePlus Hot Patch

      This following uses the original file foo.c and patch file bar.c as an example to describe how to use the LibcarePlus hot patch.


      Before using the LibcarePlus hot patch, prepare the original executable file foo and hot patch file foo.kpatch.

      Loading the Hot Patch

      The procedure for applying the LibcarePlus hot patch is as follows:

      1. In the first shell window, run the executable file to be patched:

        $ ./lpmake/foo
        Hello world!
        Hello world!
        Hello world!
      2. In the second shell window, run the libcare-ctl command to apply the hot patch:

        libcare-ctl -v patch -p $(pidof foo) ./patchroot/BuildID.kpatch

        If the hot patch is applied successfully, the following information is displayed in the second shell window:

        1 patch hunk(s) have been successfully applied to PID '10999'

        The following information is displayed for the target process running in the first shell window:

        Hello world!
        Hello world!
        Hello world being patched!
        Hello world being patched!

      Querying a Hot Patch

      The procedure for querying a LibcarePlus hot patch is as follows:

      1. Run the following command in the second shell window:

        libcare-ctl info -p $(pidof foo)

        If a hot patch is installed, the following information is displayed in the second shell window:

        Pid:                      551763
        Target:                   foo
        Build id:                 df05a25bdadd282812d3ee5f0a460e69038575de
        Applied patch number:     1
        Patch id:                 0001

      Uninstalling the Hot Patch

      The procedure for uninstalling the LibcarePlus hot patch is as follows:

      1. Run the following command in the second shell window:

        libcare-ctl unpatch -p $(pidof foo) -i 0001

        If the hot patch is uninstalled successfully, the following information is displayed in the second shell window:

        1 patch hunk(s) were successfully cancelled from PID '10999'
      2. The following information is displayed for the target process running in the first shell window:

        Hello world being patched!
        Hello world being patched!
        Hello world!
        Hello world!

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      Bug Type
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