[PATCH 1/1] kexec: provide a memfd_create() wrapper if not present in libc

[Dave Young]

On Sun, 24 Sept 2023 at 00:47, Julien Olivain <ju.o at free.fr> wrote:
>
> Commit 714fa115 "kexec/arm64: Simplify the code for zImage" introduced
> a use of the memfd_create() system call, included in version
> kexec-tools v2.0.27.
>
> This system call was introduced in kernel commit [1], first included
> in kernel v3.17 (released on 2014-10-05).
>
> The memfd_create() glibc wrapper function was added much later in
> commit [2], first included in glibc version 2.27 (released on
> 2018-02-01).
>
> This direct use memfd_create() introduced a requirement on
> Kernel >= 3.17 and glibc >= 2.27.
>
> There is old toolchains like [3] for example (which ships gcc 7.3.1,
> glibc 2.25 and includes kernel v4.10 headers), that can still be used
> to build newer kernels. Even if such toolchains can be seen as
> outdated, they are is still claimed as supported by recent kernel.
> For example, Kernel v6.5.5 has a requirement on gcc version 5.1 and
> greater. See [4].
>
> Moreover, kexec-tools <= 2.0.26 could be compiled using recent
> toolchains with alternative libc (e.g. uclibc-ng, musl) which are not
> providing the memfd_create() wrapper.
>
> When compiling kexec-tools v2.0.27 with a toolchain not providing the
> memfd_create() syscall wrapper, the compilation fail with message:
>
>     kexec/kexec.c: In function 'copybuf_memfd':
>     kexec/kexec.c:645:7: warning: implicit declaration of function 'memfd_create'; did you mean 'SYS_memfd_create'? [-Wimplicit-function-declaration]
>       fd = memfd_create("kernel", MFD_ALLOW_SEALING);
>            ^~~~~~~~~~~~
>            SYS_memfd_create
>     kexec/kexec.c:645:30: error: 'MFD_ALLOW_SEALING' undeclared (first use in this function); did you mean '_PC_ALLOC_SIZE_MIN'?
>       fd = memfd_create("kernel", MFD_ALLOW_SEALING);
>                                   ^~~~~~~~~~~~~~~~~
>                                   _PC_ALLOC_SIZE_MIN
>
> In order to let kexec-tools compile in a wider range of configurations,
> this commit adds a memfd_create() function check in autoconf configure
> script, and adds a system call wrapper which will be used if the
> function is not available. With this commit, the environment
> requirement is relaxed to only kernel >= v3.17.
>
> Note: this issue was found in kexec-tools integration in Buildroot [5]
> using the command "utils/test-pkg -a -p kexec", which tests many
> toolchain/arch combinations.

I guess maybe the test was done on non x86 arch,  when I tried to
build on old versions
I got another failure of lacking "getrandom".    Only quickly did a
build test with commenting out
the getrandom code, the build passed with your patch.

Thanks
Dave