[PATCH 1/3] tools/nolibc: x86-64: Fix startup code bug

[PATCH 1/3] tools/nolibc: x86-64: Fix startup code bug

[Willy Tarreau]

From: Ammar Faizi <ammar.faizi@students.amikom.ac.id>

Before this patch, the `_start` function looks like this:
```
0000000000001170 <_start>:
    1170:	pop    %rdi
    1171:	mov    %rsp,%rsi
    1174:	lea    0x8(%rsi,%rdi,8),%rdx
    1179:	and    $0xfffffffffffffff0,%rsp
    117d:	sub    $0x8,%rsp
    1181:	call   1000 <main>
    1186:	movzbq %al,%rdi
    118a:	mov    $0x3c,%rax
    1191:	syscall
    1193:	hlt
    1194:	data16 cs nopw 0x0(%rax,%rax,1)
    119f:	nop
```
Note the "and" to %rsp with $-16, it makes the %rsp be 16-byte aligned,
but then there is a "sub" with $0x8 which makes the %rsp no longer
16-byte aligned, then it calls main. That's the bug!

What actually the x86-64 System V ABI mandates is that right before the
"call", the %rsp must be 16-byte aligned, not after the "call". So the
"sub" with $0x8 here breaks the alignment. Remove it.

An example where this rule matters is when the callee needs to align
its stack at 16-byte for aligned move instruction, like `movdqa` and
`movaps`. If the callee can't align its stack properly, it will result
in segmentation fault.

x86-64 System V ABI also mandates the deepest stack frame should be
zero. Just to be safe, let's zero the %rbp on startup as the content
of %rbp may be unspecified when the program starts. Now it looks like
this:
```
0000000000001170 <_start>:
    1170:	pop    %rdi
    1171:	mov    %rsp,%rsi
    1174:	lea    0x8(%rsi,%rdi,8),%rdx
    1179:	xor    %ebp,%ebp                # zero the %rbp
    117b:	and    $0xfffffffffffffff0,%rsp # align the %rsp
    117f:	call   1000 <main>
    1184:	movzbq %al,%rdi
    1188:	mov    $0x3c,%rax
    118f:	syscall
    1191:	hlt
    1192:	data16 cs nopw 0x0(%rax,%rax,1)
    119d:	nopl   (%rax)
```

Cc: Bedirhan KURT <windowz414@gnuweeb.org>
Cc: Louvian Lyndal <louvianlyndal@gmail.com>
Reported-by: Peter Cordes <peter@cordes.ca>
Signed-off-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
[wt: I did this on purpose due to a misunderstanding of the spec, other
     archs will thus have to be rechecked, particularly i386]
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 tools/include/nolibc/nolibc.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 3430667b0d24..96b6d56acb57 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -399,14 +399,20 @@ struct stat {
 })
 
 /* startup code */
+/*
+ * x86-64 System V ABI mandates:
+ * 1) %rsp must be 16-byte aligned right before the function call.
+ * 2) The deepest stack frame should be zero (the %rbp).
+ *
+ */
 asm(".section .text\n"
     ".global _start\n"
     "_start:\n"
     "pop %rdi\n"                // argc   (first arg, %rdi)
     "mov %rsp, %rsi\n"          // argv[] (second arg, %rsi)
     "lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx)
-    "and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned when
-    "sub $8, %rsp\n"            // entering the callee
+    "xor %ebp, %ebp\n"          // zero the stack frame
+    "and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned before call
     "call main\n"               // main() returns the status code, we'll exit with it.
     "movzb %al, %rdi\n"         // retrieve exit code from 8 lower bits
     "mov $60, %rax\n"           // NR_exit == 60
-- 
2.17.5

[PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

[Willy Tarreau]

After re-checking in the spec and comparing stack offsets with glibc,
The last pushed argument must be 16-byte aligned (i.e. aligned before the
call) so that in the callee esp+4 is multiple of 16, so the principle is
the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that
32-bit code using SSE2 or MMX could have been affected. In addition the
frame pointer ought to be zero at the deepest level.

Link: https://gitlab.com/x86-psABIs/i386-ABI/-/wikis/Intel386-psABI
Cc: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 tools/include/nolibc/nolibc.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 96b6d56acb57..7f300dc379e7 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -583,13 +583,21 @@ struct sys_stat_struct {
 })
 
 /* startup code */
+/*
+ * i386 System V ABI mandates:
+ * 1) last pushed argument must be 16-byte aligned.
+ * 2) The deepest stack frame should be set to zero
+ *
+ */
 asm(".section .text\n"
     ".global _start\n"
     "_start:\n"
     "pop %eax\n"                // argc   (first arg, %eax)
     "mov %esp, %ebx\n"          // argv[] (second arg, %ebx)
     "lea 4(%ebx,%eax,4),%ecx\n" // then a NULL then envp (third arg, %ecx)
-    "and $-16, %esp\n"          // x86 ABI : esp must be 16-byte aligned when
+    "xor %ebp, %ebp\n"          // zero the stack frame
+    "and $-16, %esp\n"          // x86 ABI : esp must be 16-byte aligned before
+    "sub $4, %esp\n"            // the call instruction (args are aligned)
     "push %ecx\n"               // push all registers on the stack so that we
     "push %ebx\n"               // support both regparm and plain stack modes
     "push %eax\n"
-- 
2.17.5

RE: [PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

[David Laight]

From: Willy Tarreau
> Sent: 24 October 2021 18:28
> 
> After re-checking in the spec and comparing stack offsets with glibc,
> The last pushed argument must be 16-byte aligned (i.e. aligned before the
> call) so that in the callee esp+4 is multiple of 16, so the principle is
> the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that
> 32-bit code using SSE2 or MMX could have been affected. In addition the
> frame pointer ought to be zero at the deepest level.
> 
...
>  /* startup code */
> +/*
> + * i386 System V ABI mandates:
> + * 1) last pushed argument must be 16-byte aligned.
> + * 2) The deepest stack frame should be set to zero

I'm pretty sure that the historic SYSV i386 ABI only every required
4-byte alignment for the stack.

At some point it got 'randomly' changed to 16-byte.
I don't think this happened until after compiler support for SSE2
intrinsics was added.
ISTR the NetBSD found that it was gcc that moved the goalposts.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

[Willy Tarreau]

On Mon, Oct 25, 2021 at 07:46:11AM +0000, David Laight wrote:
> From: Willy Tarreau
> > Sent: 24 October 2021 18:28
> > 
> > After re-checking in the spec and comparing stack offsets with glibc,
> > The last pushed argument must be 16-byte aligned (i.e. aligned before the
> > call) so that in the callee esp+4 is multiple of 16, so the principle is
> > the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that
> > 32-bit code using SSE2 or MMX could have been affected. In addition the
> > frame pointer ought to be zero at the deepest level.
> > 
> ...
> >  /* startup code */
> > +/*
> > + * i386 System V ABI mandates:
> > + * 1) last pushed argument must be 16-byte aligned.
> > + * 2) The deepest stack frame should be set to zero
> 
> I'm pretty sure that the historic SYSV i386 ABI only every required
> 4-byte alignment for the stack.
> 
> At some point it got 'randomly' changed to 16-byte.
> I don't think this happened until after compiler support for SSE2
> intrinsics was added.

It's very possible because I've done a number of tests and noticed
that in some cases the called functions' stack doesn't seem to be
more than 4-aligned. However the deepest function in the stack starts
with an aligned stack so I prefer to follow this same rule.

Willy

RE: [PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

[David Laight]

From: Willy Tarreau
> Sent: 25 October 2021 09:06
> 
> On Mon, Oct 25, 2021 at 07:46:11AM +0000, David Laight wrote:
> > From: Willy Tarreau
> > > Sent: 24 October 2021 18:28
> > >
> > > After re-checking in the spec and comparing stack offsets with glibc,
> > > The last pushed argument must be 16-byte aligned (i.e. aligned before the
> > > call) so that in the callee esp+4 is multiple of 16, so the principle is
> > > the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that
> > > 32-bit code using SSE2 or MMX could have been affected. In addition the
> > > frame pointer ought to be zero at the deepest level.
> > >
> > ...
> > >  /* startup code */
> > > +/*
> > > + * i386 System V ABI mandates:
> > > + * 1) last pushed argument must be 16-byte aligned.
> > > + * 2) The deepest stack frame should be set to zero
> >
> > I'm pretty sure that the historic SYSV i386 ABI only every required
> > 4-byte alignment for the stack.
> >
> > At some point it got 'randomly' changed to 16-byte.
> > I don't think this happened until after compiler support for SSE2
> > intrinsics was added.
> 
> It's very possible because I've done a number of tests and noticed
> that in some cases the called functions' stack doesn't seem to be
> more than 4-aligned. However the deepest function in the stack starts
> with an aligned stack so I prefer to follow this same rule.

Any call through asm is unlikely to maintain the 16 byte alignment.
But yes, starting off on the required (by gcc) alignment does no harm.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

[PATCH 3/3] tools/nolibc: fix incorrect truncation of exit code

[Willy Tarreau]

Ammar Faizi reported that our exit code handling is wrong. We truncate
it to the lowest 8 bits but the syscall itself is expected to take a
regular 32-bit signed integer, not an unsigned char. It's the kernel
that later truncates it to the lowest 8 bits. The difference is visible
in strace, where the program below used to show exit(255) instead of
exit(-1):

  int main(void)
  {
        return -1;
  }

This patch applies the fix to all archs. x86_64, i386, arm64, armv7 and
mips were all tested and confirmed to work fine now. Risc-v was not
tested but the change is trivial and exactly the same as for other archs.

Reported-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
 tools/include/nolibc/nolibc.h | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 7f300dc379e7..3e2c6f2ed587 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -414,7 +414,7 @@ asm(".section .text\n"
     "xor %ebp, %ebp\n"          // zero the stack frame
     "and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned before call
     "call main\n"               // main() returns the status code, we'll exit with it.
-    "movzb %al, %rdi\n"         // retrieve exit code from 8 lower bits
+    "mov %eax, %edi\n"          // retrieve exit code (32 bit)
     "mov $60, %rax\n"           // NR_exit == 60
     "syscall\n"                 // really exit
     "hlt\n"                     // ensure it does not return
@@ -602,9 +602,9 @@ asm(".section .text\n"
     "push %ebx\n"               // support both regparm and plain stack modes
     "push %eax\n"
     "call main\n"               // main() returns the status code in %eax
-    "movzbl %al, %ebx\n"        // retrieve exit code from lower 8 bits
-    "movl   $1, %eax\n"         // NR_exit == 1
-    "int    $0x80\n"            // exit now
+    "mov %eax, %ebx\n"          // retrieve exit code (32-bit int)
+    "movl $1, %eax\n"           // NR_exit == 1
+    "int $0x80\n"               // exit now
     "hlt\n"                     // ensure it does not
     "");
 
@@ -788,7 +788,6 @@ asm(".section .text\n"
     "and %r3, %r1, $-8\n"         // AAPCS : sp must be 8-byte aligned in the
     "mov %sp, %r3\n"              //         callee, an bl doesn't push (lr=pc)
     "bl main\n"                   // main() returns the status code, we'll exit with it.
-    "and %r0, %r0, $0xff\n"       // limit exit code to 8 bits
     "movs r7, $1\n"               // NR_exit == 1
     "svc $0x00\n"
     "");
@@ -985,7 +984,6 @@ asm(".section .text\n"
     "add x2, x2, x1\n"            //           + argv
     "and sp, x1, -16\n"           // sp must be 16-byte aligned in the callee
     "bl main\n"                   // main() returns the status code, we'll exit with it.
-    "and x0, x0, 0xff\n"          // limit exit code to 8 bits
     "mov x8, 93\n"                // NR_exit == 93
     "svc #0\n"
     "");
@@ -1190,7 +1188,7 @@ asm(".section .text\n"
     "addiu $sp,$sp,-16\n"         // the callee expects to save a0..a3 there!
     "jal main\n"                  // main() returns the status code, we'll exit with it.
     "nop\n"                       // delayed slot
-    "and $a0, $v0, 0xff\n"        // limit exit code to 8 bits
+    "move $a0, $v0\n"             // retrieve 32-bit exit code from v0
     "li $v0, 4001\n"              // NR_exit == 4001
     "syscall\n"
     ".end __start\n"
@@ -1388,7 +1386,6 @@ asm(".section .text\n"
     "add   a2,a2,a1\n"           //             + argv
     "andi  sp,a1,-16\n"          // sp must be 16-byte aligned
     "call  main\n"               // main() returns the status code, we'll exit with it.
-    "andi  a0, a0, 0xff\n"       // limit exit code to 8 bits
     "li a7, 93\n"                // NR_exit == 93
     "ecall\n"
     "");
-- 
2.17.5