Re: [PATCH] linux-user: Add AT_EXECFN and AT_EXECFD auxval

[Lirong Yuan <yuanzi@google.com>]

On Mon, Mar 2, 2020 at 6:39 AM Laurent Vivier <laurent@vivier.eu> wrote:
>
> Le 21/02/2020 à 21:28, Lirong Yuan a écrit :
> > This change adds the support for AT_EXECFN and AT_EXECFD auxval.
>
> Why do we need AT_EXECFD?
>
> AT_EXECFD is normally only used with binfmt_misc so I don't see any use
> cases for it with QEMU.
>
> For AT_EXECFN, according to kernel commit
>
>   651910874633 execve filename: document and export via auxiliary vector
>
> It sould be like readlink("/proc/self/exe",), and thus I think we should
> use realpath() like we have in syscall.c for TARGET_NR_readlink:
>
>  8843     case TARGET_NR_readlink:
> ...
>  8854                 char real[PATH_MAX], *temp;
>  8855                 temp = realpath(exec_path, real);
> ...
>
> Thanks,
> Laurent
>
> >
> > Signed-off-by: Lirong Yuan <yuanzi@google.com>
> > ---
> >  linux-user/elfload.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> > index f3080a1635..7e0f3042f1 100644
> > --- a/linux-user/elfload.c
> > +++ b/linux-user/elfload.c
> > @@ -1568,7 +1568,7 @@ struct exec
> >                                   ~(abi_ulong)(TARGET_ELF_EXEC_PAGESIZE-1))
> >  #define TARGET_ELF_PAGEOFFSET(_v) ((_v) & (TARGET_ELF_EXEC_PAGESIZE-1))
> >
> > -#define DLINFO_ITEMS 15
> > +#define DLINFO_ITEMS 17
> >
> >  static inline void memcpy_fromfs(void * to, const void * from, unsigned long n)
> >  {
> > @@ -1888,11 +1888,14 @@ static abi_ulong loader_build_fdpic_loadmap(struct image_info *info, abi_ulong s
> >      return sp;
> >  }
> >
> > -static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
> > +static abi_ulong create_elf_tables(struct linux_binprm *bprm,
> >                                     struct elfhdr *exec,
> >                                     struct image_info *info,
> >                                     struct image_info *interp_info)
> >  {
> > +    abi_ulong p = bprm->p;
> > +    int argc = bprm->argc;
> > +    int envc = bprm->envc;
> >      abi_ulong sp;
> >      abi_ulong u_argc, u_argv, u_envp, u_auxv;
> >      int size;
> > @@ -2032,6 +2035,8 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
> >      NEW_AUX_ENT(AT_CLKTCK, (abi_ulong) sysconf(_SC_CLK_TCK));
> >      NEW_AUX_ENT(AT_RANDOM, (abi_ulong) u_rand_bytes);
> >      NEW_AUX_ENT(AT_SECURE, (abi_ulong) qemu_getauxval(AT_SECURE));
> > +    NEW_AUX_ENT(AT_EXECFN, info->file_string);
> > +    NEW_AUX_ENT(AT_EXECFD, bprm->fd);
> >
> >  #ifdef ELF_HWCAP2
> >      NEW_AUX_ENT(AT_HWCAP2, (abi_ulong) ELF_HWCAP2);
> > @@ -2870,8 +2875,8 @@ int load_elf_binary(struct linux_binprm *bprm, struct image_info *info)
> >  #endif
> >      }
> >
> > -    bprm->p = create_elf_tables(bprm->p, bprm->argc, bprm->envc, &elf_ex,
> > -                                info, (elf_interpreter ? &interp_info : NULL));
> > +    bprm->p = create_elf_tables(bprm, &elf_ex, info,
> > +                                (elf_interpreter ? &interp_info : NULL));
> >      info->start_stack = bprm->p;
> >
> >      /* If we have an interpreter, set that as the program's entry point.
> >
>

Hi Laurent,

I added support for AT_EXECFD because I thought it might be useful to
implement all types that getauxval could take as an argument.
Would you prefer that it be removed?

For AT_EXECFN, there are two questions that we considered:
1) What should it return?
Since QEMU is emulating running the guest program, the function should
return the file name of the guest program (info->file_string), rather
than the QEMU program itself, which we get from
qemu_getauxval(AT_EXECFN).

2) Should it return the full path or as is?
We tested the behavior of getauxval with a simple test program on
Linux, and it turned out that it returned file path as is. For
example,
$ ./test
getauxval(AT_EXECFN): ./test
$ /usr/local/home/tmp/test
getauxval(AT_EXECFN): /usr/local/home/tmp/test

It would seem that the current solution is working as intended.

Thanks,
Lirong