Re: git: 64ba1f4cf3a6 - main - rtld: Implement LD_SHOW_AUXV

On 13 Nov 2021, at 17:33, Konstantin Belousov <kib@FreeBSD.org> wrote:
> 
> The branch main has been updated by kib:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=64ba1f4cf3a6847a1dacf4bab0409d94898fa168
> 
> commit 64ba1f4cf3a6847a1dacf4bab0409d94898fa168
> Author:     Konstantin Belousov <kib@FreeBSD.org>
> AuthorDate: 2021-11-13 01:18:13 +0000
> Commit:     Konstantin Belousov <kib@FreeBSD.org>
> CommitDate: 2021-11-13 17:33:13 +0000
> 
>    rtld: Implement LD_SHOW_AUXV
> 
>    It dumps auxv as seen by interpreter, right before starting any user
>    code.
> 
>    Copied from:    glibc
>    Sponsored by:   The FreeBSD Foundation
>    MFC after:      1 week
> ---
> libexec/rtld-elf/rtld.1 |  7 +++++-
> libexec/rtld-elf/rtld.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 73 insertions(+), 1 deletion(-)
> 
> diff --git a/libexec/rtld-elf/rtld.1 b/libexec/rtld-elf/rtld.1
> index 187dc105667a..66aa2bdabd17 100644
> --- a/libexec/rtld-elf/rtld.1
> +++ b/libexec/rtld-elf/rtld.1
> @@ -28,7 +28,7 @@
> .\"
> .\" $FreeBSD$
> .\"
> -.Dd August 15, 2021
> +.Dd November 13, 2021
> .Dt RTLD 1
> .Os
> .Sh NAME
> @@ -309,6 +309,11 @@ will process the filtee dependencies of the loaded objects immediately,
> instead of postponing it until required.
> Normally, the filtees are opened at the time of the first symbol resolution
> from the filter object.
> +.It Ev LD_SHOW_AUXV
> +If set, causes
> +.Nm
> +to dump content of the aux vector to standard output, before passing
> +control to any user code.
> .El
> .Sh DIRECT EXECUTION MODE
> .Nm
> diff --git a/libexec/rtld-elf/rtld.c b/libexec/rtld-elf/rtld.c
> index c173c5a6e22e..0475134b0d96 100644
> --- a/libexec/rtld-elf/rtld.c
> +++ b/libexec/rtld-elf/rtld.c
> @@ -104,6 +104,7 @@ static Obj_Entry *dlopen_object(const char *name, int fd, Obj_Entry *refobj,
> static Obj_Entry *do_load_object(int, const char *, char *, struct stat *, int);
> static int do_search_info(const Obj_Entry *obj, int, struct dl_serinfo *);
> static bool donelist_check(DoneList *, const Obj_Entry *);
> +static void dump_auxv(Elf_Auxinfo **aux_info);
> static void errmsg_restore(struct dlerror_save *);
> static struct dlerror_save *errmsg_save(void);
> static void *fill_search_info(const char *, size_t, void *);
> @@ -364,6 +365,7 @@ enum {
> 	LD_TRACE_LOADED_OBJECTS_FMT1,
> 	LD_TRACE_LOADED_OBJECTS_FMT2,
> 	LD_TRACE_LOADED_OBJECTS_ALL,
> +	LD_SHOW_AUXV,
> };
> 
> struct ld_env_var_desc {
> @@ -396,6 +398,7 @@ static struct ld_env_var_desc ld_env_vars[] = {
> 	LD_ENV_DESC(TRACE_LOADED_OBJECTS_FMT1, false),
> 	LD_ENV_DESC(TRACE_LOADED_OBJECTS_FMT2, false),
> 	LD_ENV_DESC(TRACE_LOADED_OBJECTS_ALL, false),
> +	LD_ENV_DESC(SHOW_AUXV, false),
> };
> 
> static const char *
> @@ -857,6 +860,9 @@ _rtld(Elf_Addr *sp, func_ptr_type *exit_proc, Obj_Entry **objp)
>     if (rtld_verify_versions(&list_main) == -1 && !ld_tracing)
> 	rtld_die();
> 
> +    if (ld_get_env_var(LD_SHOW_AUXV) != NULL)
> +       dump_auxv(aux_info);
> +
>     if (ld_tracing) {		/* We're done */
> 	trace_loaded_objects(obj_main);
> 	exit(0);
> @@ -6058,6 +6064,67 @@ print_usage(const char *argv0)
> 	    "  <args>    Arguments to the executed process\n", argv0);
> }
> 
> +#define	AUXFMT(at, xfmt) [at] = { .name = #at, .fmt = xfmt }
> +static const struct auxfmt {
> +	const char *name;
> +	const char *fmt;
> +} auxfmts[] = {
> +	AUXFMT(AT_NULL, NULL),
> +	AUXFMT(AT_IGNORE, NULL),
> +	AUXFMT(AT_EXECFD, "%d"),
> +	AUXFMT(AT_PHDR, "%p"),
> +	AUXFMT(AT_PHENT, "%u"),
> +	AUXFMT(AT_PHNUM, "%u"),
> +	AUXFMT(AT_PAGESZ, "%u"),
> +	AUXFMT(AT_BASE, "%#lx"),
> +	AUXFMT(AT_FLAGS, "%#lx"),
> +	AUXFMT(AT_ENTRY, "%p"),
> +	AUXFMT(AT_NOTELF, NULL),
> +	AUXFMT(AT_UID, "%d"),
> +	AUXFMT(AT_EUID, "%d"),
> +	AUXFMT(AT_GID, "%d"),
> +	AUXFMT(AT_EGID, "%d"),
> +	AUXFMT(AT_EXECPATH, "%s"),
> +	AUXFMT(AT_CANARY, "%p"),
> +	AUXFMT(AT_CANARYLEN, "%u"),
> +	AUXFMT(AT_OSRELDATE, "%u"),
> +	AUXFMT(AT_NCPUS, "%u"),
> +	AUXFMT(AT_PAGESIZES, "%p"),
> +	AUXFMT(AT_PAGESIZESLEN, "%u"),
> +	AUXFMT(AT_TIMEKEEP, "%p"),
> +	AUXFMT(AT_STACKPROT, "%#x"),
> +	AUXFMT(AT_EHDRFLAGS, "%#lx"),
> +	AUXFMT(AT_HWCAP, "%#lx"),
> +	AUXFMT(AT_HWCAP2, "%#lx"),
> +	AUXFMT(AT_BSDFLAGS, "%#lx"),
> +	AUXFMT(AT_ARGC, "%u"),
> +	AUXFMT(AT_ARGV, "%p"),
> +	AUXFMT(AT_ENVC, "%p"),
> +	AUXFMT(AT_ENVV, "%p"),
> +	AUXFMT(AT_PS_STRINGS, "%p"),
> +	AUXFMT(AT_FXRNG, "%p"),
> +};
> +
> +static void
> +dump_auxv(Elf_Auxinfo **aux_info)
> +{
> +	Elf_Auxinfo *auxp;
> +	const struct auxfmt *fmt;
> +	int i;
> +
> +	for (i = 0; i < AT_COUNT; i++) {
> +		auxp = aux_info[i];
> +		if (auxp == NULL)
> +			continue;
> +		fmt = &auxfmts[i];
> +		if (fmt->fmt == NULL)
> +			continue;
> +		rtld_fdprintf(STDOUT_FILENO, "%s:\t", fmt->name);
> +		rtld_fdprintfx(STDOUT_FILENO, fmt->fmt, auxp->a_un.a_ptr);
> +		rtld_fdprintf(STDOUT_FILENO, "\n");

This is undefined behaviour, breaks CHERI, and totally unnecessary. You
have a handful of cases here, just make an enum and have separate
rtld_fdprintf calls.

Also the table itself is brittle, there’s nothing checking that the
order perfectly matches up with the defines in the header. Why not use
designated initialisers to ensure that the right values are in the
right entries (and handle the possibility that name might be NULL)?

Jess