[PATCH v3 0/2] Args adjustment with ./ld.so exe [BZ #23293]

[PATCH v3 0/2] Args adjustment with ./ld.so exe [BZ #23293]

[Szabolcs Nagy]

Trying to make the ld.so start code more generic and less error prone.

Only fixing aarch64 here, other targets have to opt-in by defining
DL_NEED_START_ARGS_ADJUST.

Szabolcs Nagy (2):
  aarch64: Use generic argv adjustment in ld.so [BZ #23293]
  aarch64: Move ld.so _start to separate file and drop _dl_skip_args

 elf/rtld.c                          | 58 ++++++++++++++++++++++
 sysdeps/aarch64/Makefile            |  1 +
 sysdeps/aarch64/dl-machine.h        | 77 +----------------------------
 sysdeps/aarch64/dl-start.S          | 53 ++++++++++++++++++++
 sysdeps/aarch64/dl-sysdep.h         |  2 +-
 sysdeps/generic/ldsodefs.h          |  3 ++
 sysdeps/unix/sysv/linux/dl-sysdep.c | 10 ++++
 7 files changed, 128 insertions(+), 76 deletions(-)
 create mode 100644 sysdeps/aarch64/dl-start.S

-- 
2.25.1

[PATCH v3 1/2] aarch64: Use generic argv adjustment in ld.so [BZ #23293]

[Szabolcs Nagy]

When an executable is invoked as

  ./ld.so [ld.so-args] ./exe [exe-args]

then the argv is adujusted in ld.so before calling the entry point of
the executable so ld.so args are not visible to it.  On most targets
this requires moving argv, env and auxv on the stack to ensure correct
stack alignment at the entry point.  This had several issues:

- The code for this adjustment on the stack is written in asm as part
  of the target specific ld.so _start code which is hard to maintain.

- The adjustment is done after _dl_start returns, where it's too late
  to update GLRO(dl_auxv), as it is already readonly, so it points to
  memory that was clobbered by the adjustment. This is bug 23293.

- _environ is also wrong in ld.so after the adjustment, but it is
  likely not used after _dl_start returns so this is not user visible.

- _dl_argv was updated, but for this it was moved out of relro, which
  changes security properties across targets unnecessarily.

This patch introduces a generic _dl_start_args_adjust function that
handles the argument adjustments after ld.so processed its own args
and before relro protection is applied.  It sets _dl_skip_args to 0 so
the existing adjustment in asm is not invoked.  Each target has to
opt-in to use this new adjustment since some targets don't need it.
Once all targets are updated, _dl_argv declaration can be simplified.

A new _dl_start_argptr was introduced because the original sp is not
passed to dl_main which now has to do the adjustments.

--
v2:
- use p != NULL, and a_type != AT_NULL
- remove the confusing paragraph from the commit message.
---
 elf/rtld.c                          | 58 +++++++++++++++++++++++++++++
 sysdeps/aarch64/dl-sysdep.h         |  2 +-
 sysdeps/generic/ldsodefs.h          |  3 ++
 sysdeps/unix/sysv/linux/dl-sysdep.c | 10 +++++
 4 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/elf/rtld.c b/elf/rtld.c
index 19e328f89e..bc4d59f5d6 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -1311,6 +1311,60 @@ rtld_setup_main_map (struct link_map *main_map)
   return has_interp;
 }
 
+#ifdef DL_NEED_START_ARGS_ADJUST
+static void
+_dl_start_args_adjust (void)
+{
+  void **sp;
+  void **p;
+  long argc;
+  char **argv;
+  ElfW(auxv_t) *auxv;
+
+  if (_dl_skip_args == 0)
+    return;
+
+  sp = _dl_start_argptr;
+
+  /* Adjust argc on stack.  */
+  argc = (long) sp[0] - _dl_skip_args;
+  sp[0] = (void *) argc;
+
+  argv = (char **) (sp + 1); /* Necessary aliasing violation.  */
+  p = sp + _dl_skip_args;
+  /* Shuffle argv down.  */
+  do
+    *++sp = *++p;
+  while (*p != NULL);
+
+  /* Shuffle envp down.  */
+  do
+    *++sp = *++p;
+  while (*p != NULL);
+
+  auxv = (ElfW(auxv_t) *) (sp + 1); /* Necessary aliasing violation.  */
+  /* Shuffle auxv down. */
+  void *a, *b; /* Use a pair of pointers for an auxv entry.  */
+  unsigned long a_type;
+  do
+    {
+      a_type = ((ElfW(auxv_t) *) (p + 1))->a_type;
+      a = *++p;
+      b = *++p;
+      *++sp = a;
+      *++sp = b;
+    }
+  while (a_type != AT_NULL);
+
+  /* Update globals in rtld.  */
+  _dl_argv = argv;
+  _environ = argv + argc + 1;
+  GLRO(dl_auxv) = auxv;
+  /* No longer need to skip args.  */
+  _dl_skip_args = 0;
+}
+#endif
+
 static void
 dl_main (const ElfW(Phdr) *phdr,
 	 ElfW(Word) phnum,
@@ -1615,6 +1669,10 @@ dl_main (const ElfW(Phdr) *phdr,
       /* Set the argv[0] string now that we've processed the executable.  */
       if (argv0 != NULL)
         _dl_argv[0] = argv0;
+#ifdef DL_NEED_START_ARGS_ADJUST
+      /* Adjust arguments for the application entry point.  */
+      _dl_start_args_adjust ();
+#endif
     }
   else
     {
diff --git a/sysdeps/aarch64/dl-sysdep.h b/sysdeps/aarch64/dl-sysdep.h
index 667786671c..1df4c2c528 100644
--- a/sysdeps/aarch64/dl-sysdep.h
+++ b/sysdeps/aarch64/dl-sysdep.h
@@ -20,6 +20,6 @@
 
 /* _dl_argv cannot be attribute_relro, because _dl_start_user
    might write into it after _dl_start returns.  */
-#define DL_ARGV_NOT_RELRO 1
+#define DL_NEED_START_ARGS_ADJUST 1
 
 #define DL_EXTERN_PROTECTED_DATA
diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 29f005499b..f322d36570 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -785,6 +785,9 @@ extern unsigned int _dl_skip_args attribute_hidden
      attribute_relro
 # endif
      ;
+# ifdef DL_NEED_START_ARGS_ADJUST
+extern void **_dl_start_argptr attribute_hidden attribute_relro;
+# endif
 #endif
 #define rtld_progname _dl_argv[0]
 
diff --git a/sysdeps/unix/sysv/linux/dl-sysdep.c b/sysdeps/unix/sysv/linux/dl-sysdep.c
index c90f109b11..66f003e2a3 100644
--- a/sysdeps/unix/sysv/linux/dl-sysdep.c
+++ b/sysdeps/unix/sysv/linux/dl-sysdep.c
@@ -58,6 +58,12 @@ void *__libc_stack_end attribute_relro = NULL;
 rtld_hidden_data_def(__libc_stack_end)
 void *_dl_random attribute_relro = NULL;
 
+#ifdef DL_NEED_START_ARGS_ADJUST
+/* Original sp at ELF entry, used when rtld is executed explicitly
+   and needs to adjust arg components for the actual application.  */
+void **_dl_start_argptr attribute_hidden attribute_relro = NULL;
+#endif
+
 #ifndef DL_STACK_END
 # define DL_STACK_END(cookie) ((void *) (cookie))
 #endif
@@ -114,6 +120,10 @@ _dl_sysdep_start (void **start_argptr,
 
   __brk (0);			/* Initialize the break.  */
 
+#ifdef DL_NEED_START_ARGS_ADJUST
+  _dl_start_argptr = start_argptr;
+#endif
+
 #ifdef DL_PLATFORM_INIT
   DL_PLATFORM_INIT;
 #endif
-- 
2.25.1

Re: [PATCH v3 1/2] aarch64: Use generic argv adjustment in ld.so [BZ #23293]

[Florian Weimer]

* Szabolcs Nagy via Libc-alpha:

> When an executable is invoked as
>
>   ./ld.so [ld.so-args] ./exe [exe-args]
>
> then the argv is adujusted in ld.so before calling the entry point of
> the executable so ld.so args are not visible to it.  On most targets
> this requires moving argv, env and auxv on the stack to ensure correct
> stack alignment at the entry point.  This had several issues:
>
> - The code for this adjustment on the stack is written in asm as part
>   of the target specific ld.so _start code which is hard to maintain.
>
> - The adjustment is done after _dl_start returns, where it's too late
>   to update GLRO(dl_auxv), as it is already readonly, so it points to
>   memory that was clobbered by the adjustment. This is bug 23293.
>
> - _environ is also wrong in ld.so after the adjustment, but it is
>   likely not used after _dl_start returns so this is not user visible.
>
> - _dl_argv was updated, but for this it was moved out of relro, which
>   changes security properties across targets unnecessarily.
>
> This patch introduces a generic _dl_start_args_adjust function that
> handles the argument adjustments after ld.so processed its own args
> and before relro protection is applied.  It sets _dl_skip_args to 0 so
> the existing adjustment in asm is not invoked.  Each target has to
> opt-in to use this new adjustment since some targets don't need it.
> Once all targets are updated, _dl_argv declaration can be simplified.
>
> A new _dl_start_argptr was introduced because the original sp is not
> passed to dl_main which now has to do the adjustments.
>
> --
> v2:
> - use p != NULL, and a_type != AT_NULL
> - remove the confusing paragraph from the commit message.

Looks good to me.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian

Re: [PATCH v3 1/2] aarch64: Use generic argv adjustment in ld.so [BZ #23293]

[Szabolcs Nagy]

The 05/03/2022 13:53, Florian Weimer via Libc-alpha wrote:
> * Szabolcs Nagy via Libc-alpha:
> 
> > When an executable is invoked as
> >
> >   ./ld.so [ld.so-args] ./exe [exe-args]
> >
> > then the argv is adujusted in ld.so before calling the entry point of
> > the executable so ld.so args are not visible to it.  On most targets
> > this requires moving argv, env and auxv on the stack to ensure correct
> > stack alignment at the entry point.  This had several issues:
> >
> > - The code for this adjustment on the stack is written in asm as part
> >   of the target specific ld.so _start code which is hard to maintain.
> >
> > - The adjustment is done after _dl_start returns, where it's too late
> >   to update GLRO(dl_auxv), as it is already readonly, so it points to
> >   memory that was clobbered by the adjustment. This is bug 23293.
> >
> > - _environ is also wrong in ld.so after the adjustment, but it is
> >   likely not used after _dl_start returns so this is not user visible.
> >
> > - _dl_argv was updated, but for this it was moved out of relro, which
> >   changes security properties across targets unnecessarily.
> >
> > This patch introduces a generic _dl_start_args_adjust function that
> > handles the argument adjustments after ld.so processed its own args
> > and before relro protection is applied.  It sets _dl_skip_args to 0 so
> > the existing adjustment in asm is not invoked.  Each target has to
> > opt-in to use this new adjustment since some targets don't need it.
> > Once all targets are updated, _dl_argv declaration can be simplified.
> >
> > A new _dl_start_argptr was introduced because the original sp is not
> > passed to dl_main which now has to do the adjustments.
> >
> > --
> > v2:
> > - use p != NULL, and a_type != AT_NULL
> > - remove the confusing paragraph from the commit message.
> 
> Looks good to me.
> 
> Reviewed-by: Florian Weimer <fweimer@redhat.com>

thanks, meanwhile i started working on v4.

that always does the new adjustment (even on targets that don't
require it) since ld.so invocation does not have to be optimal
but it's better to have consistent behaviour.

then the bug is fixed everywhere (not just opt-in).

is that an acceptable approach?

Re: [PATCH v3 1/2] aarch64: Use generic argv adjustment in ld.so [BZ #23293]

[Florian Weimer]

* Szabolcs Nagy:

> The 05/03/2022 13:53, Florian Weimer via Libc-alpha wrote:
>> * Szabolcs Nagy via Libc-alpha:
>> 
>> > When an executable is invoked as
>> >
>> >   ./ld.so [ld.so-args] ./exe [exe-args]
>> >
>> > then the argv is adujusted in ld.so before calling the entry point of
>> > the executable so ld.so args are not visible to it.  On most targets
>> > this requires moving argv, env and auxv on the stack to ensure correct
>> > stack alignment at the entry point.  This had several issues:
>> >
>> > - The code for this adjustment on the stack is written in asm as part
>> >   of the target specific ld.so _start code which is hard to maintain.
>> >
>> > - The adjustment is done after _dl_start returns, where it's too late
>> >   to update GLRO(dl_auxv), as it is already readonly, so it points to
>> >   memory that was clobbered by the adjustment. This is bug 23293.
>> >
>> > - _environ is also wrong in ld.so after the adjustment, but it is
>> >   likely not used after _dl_start returns so this is not user visible.
>> >
>> > - _dl_argv was updated, but for this it was moved out of relro, which
>> >   changes security properties across targets unnecessarily.
>> >
>> > This patch introduces a generic _dl_start_args_adjust function that
>> > handles the argument adjustments after ld.so processed its own args
>> > and before relro protection is applied.  It sets _dl_skip_args to 0 so
>> > the existing adjustment in asm is not invoked.  Each target has to
>> > opt-in to use this new adjustment since some targets don't need it.
>> > Once all targets are updated, _dl_argv declaration can be simplified.
>> >
>> > A new _dl_start_argptr was introduced because the original sp is not
>> > passed to dl_main which now has to do the adjustments.
>> >
>> > --
>> > v2:
>> > - use p != NULL, and a_type != AT_NULL
>> > - remove the confusing paragraph from the commit message.
>> 
>> Looks good to me.
>> 
>> Reviewed-by: Florian Weimer <fweimer@redhat.com>
>
> thanks, meanwhile i started working on v4.
>
> that always does the new adjustment (even on targets that don't
> require it) since ld.so invocation does not have to be optimal
> but it's better to have consistent behaviour.
>
> then the bug is fixed everywhere (not just opt-in).
>
> is that an acceptable approach?

Sorry, I would have to see the patch.  But I generally appreciate
consistency.

Thanks,
Florian

[PATCH v3 2/2] aarch64: Move ld.so _start to separate file and drop _dl_skip_args

[Szabolcs Nagy]

A separate asm file is easier to maintain than a macro that expands to
inline asm.

The RTLD_START macro is only needed now because _dl_start is local in
rtld.c, but _start has to call it, if _dl_start was made hidden then it
could be empty.

_dl_skip_args is no longer handled in the _start asm, instead we rely
on DL_NEED_START_ARGS_ADJUST.

--
v3:
- mention _dl_skip_args
v2:
- fix typo in commit message.
---
 sysdeps/aarch64/Makefile     |  1 +
 sysdeps/aarch64/dl-machine.h | 77 +-----------------------------------
 sysdeps/aarch64/dl-start.S   | 53 +++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 75 deletions(-)
 create mode 100644 sysdeps/aarch64/dl-start.S

diff --git a/sysdeps/aarch64/Makefile b/sysdeps/aarch64/Makefile
index 7183895d04..17fb1c5b72 100644
--- a/sysdeps/aarch64/Makefile
+++ b/sysdeps/aarch64/Makefile
@@ -33,6 +33,7 @@ tst-audit27-ENV = LD_AUDIT=$(objpfx)tst-auditmod27.so
 endif
 
 ifeq ($(subdir),elf)
+sysdep-rtld-routines += dl-start
 sysdep-dl-routines += tlsdesc dl-tlsdesc
 gen-as-const-headers += dl-link.sym
 
diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index b40050a981..fe120bb507 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -105,81 +105,8 @@ elf_machine_runtime_setup (struct link_map *l, struct r_scope_elem *scope[],
   return lazy;
 }
 
-/* Initial entry point for the dynamic linker. The C function
-   _dl_start is the real entry point, its return value is the user
-   program's entry point */
-#ifdef __LP64__
-# define RTLD_START RTLD_START_1 ("x", "3", "sp")
-#else
-# define RTLD_START RTLD_START_1 ("w", "2", "wsp")
-#endif
-
-
-#define RTLD_START_1(PTR, PTR_SIZE_LOG, PTR_SP) asm ("\
-.text									\n\
-.globl _start								\n\
-.type _start, %function							\n\
-.globl _dl_start_user							\n\
-.type _dl_start_user, %function						\n\
-_start:									\n\
-	// bti c							\n\
-	hint	34							\n\
-	mov	" PTR "0, " PTR_SP "					\n\
-	bl	_dl_start						\n\
-	// returns user entry point in x0				\n\
-	mov	x21, x0							\n\
-_dl_start_user:								\n\
-	// get the original arg count					\n\
-	ldr	" PTR "1, [sp]						\n\
-	// get the argv address						\n\
-	add	" PTR "2, " PTR_SP ", #(1<<"  PTR_SIZE_LOG ")		\n\
-	// get _dl_skip_args to see if we were				\n\
-	// invoked as an executable					\n\
-	adrp	x4, _dl_skip_args					\n\
-        ldr	w4, [x4, #:lo12:_dl_skip_args]				\n\
-	// do we need to adjust argc/argv				\n\
-        cmp	w4, 0							\n\
-	beq	.L_done_stack_adjust					\n\
-	// subtract _dl_skip_args from original arg count		\n\
-	sub	" PTR "1, " PTR "1, " PTR "4				\n\
-	// store adjusted argc back to stack				\n\
-	str	" PTR "1, [sp]						\n\
-	// find the first unskipped argument				\n\
-	mov	" PTR "3, " PTR "2					\n\
-	add	" PTR "4, " PTR "2, " PTR "4, lsl #" PTR_SIZE_LOG "	\n\
-	// shuffle argv down						\n\
-1:	ldr	" PTR "5, [x4], #(1<<"  PTR_SIZE_LOG ")			\n\
-	str	" PTR "5, [x3], #(1<<"  PTR_SIZE_LOG ")			\n\
-	cmp	" PTR "5, #0						\n\
-	bne	1b							\n\
-	// shuffle envp down						\n\
-1:	ldr	" PTR "5, [x4], #(1<<"  PTR_SIZE_LOG ")			\n\
-	str	" PTR "5, [x3], #(1<<"  PTR_SIZE_LOG ")			\n\
-	cmp	" PTR "5, #0						\n\
-	bne	1b							\n\
-	// shuffle auxv down						\n\
-1:	ldp	" PTR "0, " PTR "5, [x4, #(2<<"  PTR_SIZE_LOG ")]!	\n\
-	stp	" PTR "0, " PTR "5, [x3], #(2<<"  PTR_SIZE_LOG ")	\n\
-	cmp	" PTR "0, #0						\n\
-	bne	1b							\n\
-	// Update _dl_argv						\n\
-	adrp	x3, __GI__dl_argv					\n\
-	str	" PTR "2, [x3, #:lo12:__GI__dl_argv]			\n\
-.L_done_stack_adjust:							\n\
-	// compute envp							\n\
-	add	" PTR "3, " PTR "2, " PTR "1, lsl #" PTR_SIZE_LOG "	\n\
-	add	" PTR "3, " PTR "3, #(1<<"  PTR_SIZE_LOG ")		\n\
-	adrp	x16, _rtld_local					\n\
-        add	" PTR "16, " PTR "16, #:lo12:_rtld_local		\n\
-        ldr	" PTR "0, [x16]						\n\
-	bl	_dl_init						\n\
-	// load the finalizer function					\n\
-	adrp	x0, _dl_fini						\n\
-	add	" PTR "0, " PTR "0, #:lo12:_dl_fini			\n\
-	// jump to the user_s entry point				\n\
-	mov     x16, x21						\n\
-	br      x16							\n\
-");
+/* In elf/rtld.c _dl_start should be global so dl-start.S can reference it.  */
+#define RTLD_START asm (".globl _dl_start");
 
 #define elf_machine_type_class(type)					\
   ((((type) == AARCH64_R(JUMP_SLOT)					\
diff --git a/sysdeps/aarch64/dl-start.S b/sysdeps/aarch64/dl-start.S
new file mode 100644
index 0000000000..a3a57bd5a1
--- /dev/null
+++ b/sysdeps/aarch64/dl-start.S
@@ -0,0 +1,53 @@
+/* ld.so _start code.
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sysdep.h>
+
+ENTRY (_start)
+	/* Create an initial frame with 0 LR and FP */
+	cfi_undefined (x30)
+	mov	x29, #0
+	mov	x30, #0
+
+	mov	x0, sp
+	PTR_ARG (0)
+	bl	_dl_start
+	/* Returns user entry point in x0.  */
+	mov	PTR_REG (21), PTR_REG (0)
+.globl _dl_start_user
+.type _dl_start_user, %function
+_dl_start_user:
+	/* Get argc.  */
+	ldr	PTR_REG (1), [sp]
+	/* Get argv.  */
+	add	x2, sp, PTR_SIZE
+	/* Compute envp.  */
+	add	PTR_REG (3), PTR_REG (2), PTR_REG (1), lsl PTR_LOG_SIZE
+	add	PTR_REG (3), PTR_REG (3), PTR_SIZE
+	adrp	x16, _rtld_local
+	add	PTR_REG (16), PTR_REG (16), :lo12:_rtld_local
+	ldr	PTR_REG (0), [x16]
+	bl	_dl_init
+	/* Load the finalizer function.  */
+	adrp	x0, _dl_fini
+	add	PTR_REG (0), PTR_REG (0), :lo12:_dl_fini
+	/* Jump to the user's entry point.  */
+	mov     x16, x21
+	br      x16
+END (_start)
-- 
2.25.1

Re: [PATCH v3 2/2] aarch64: Move ld.so _start to separate file and drop _dl_skip_args

[Florian Weimer]

* Szabolcs Nagy via Libc-alpha:

> A separate asm file is easier to maintain than a macro that expands to
> inline asm.
>
> The RTLD_START macro is only needed now because _dl_start is local in
> rtld.c, but _start has to call it, if _dl_start was made hidden then it
> could be empty.
>
> _dl_skip_args is no longer handled in the _start asm, instead we rely
> on DL_NEED_START_ARGS_ADJUST.

This version looks okay to me.  You can probably push this as an aarch64
maintainer.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

Thanks,
Florian