Re: git: 64ba1f4cf3a6 - main - rtld: Implement LD_SHOW_AUXV
Date: Sat, 13 Nov 2021 18:29:24 UTC
On 13 Nov 2021, at 17:57, Jessica Clarke <jrtc27@FreeBSD.org> wrote:
>
> On 13 Nov 2021, at 17:54, Jessica Clarke <jrtc27@FreeBSD.org> wrote:
>>
>> 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.
In particular, ignoring CHERI, unsigned ints are sign-extended to 64
bits on MIPS and RISC-V. Thus by passing a 64-bit value but using a %u,
you are violating the calling convention. I can’t currently get GCC or
Clang to exploit the fact that varargs arguments are sign-extended, but
on MIPS, and RISC-V GCC (Clang is currently stupid and round-trips via
memory even when the va_arg calls have no branching surrounding them,
rather than just grabbing from the register) there is a redundant
sext.w that can legally be optimised out, but would be broken by this
calling convention violation.
Then CHERI makes it worse because a_ptr and a_val do not have the same
representation, although in practice I think passing a_ptr and nothing
further does end up working on CHERI-RISC-V and Morello, just not
CHERI-MIPS due to being big-endian.
Jess
>> 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)?
>
> Scratch that second part, I missed the [at] = in the macro.
>
> Jess