Re: malloc() alignment on 32 bit

[Sascha Hauer <sha@pengutronix.de>]

Hi Enrico,

On Mon, Sep 19, 2022 at 02:37:59PM +0200, Enrico Scholz wrote:
> Hello,
> 
> on an iMX6ull I stumpled across
> 
> | zstd_decomp_init:536 workspace=8ff1a004+161320
> | ERROR: initcall ubifs_init+0x1/0xc4 failed: Invalid argument
> 
> which is caused by
> 
> | static int zstd_decomp_init(void)
> |	void *wksp = malloc(wksp_size);
> | ...
> | ZSTD_DCtx* ZSTD_initStaticDCtx(void *workspace, size_t workspaceSize)
> |    if ((size_t)workspace & 7) return NULL;  /* 8-aligned */
> 
> 
> Trivial fix would be 'memalign(8, wksp_size)', but is it really ok that
> malloc() for 32 bit has only an alignment of 4?
> 
> Relevant code seems to be in common/tlsf.c
> 
> | enum tlsf_private
> | {
> | #if defined (TLSF_64BIT)
> | 	/* All allocation sizes and addresses are aligned to 8 bytes. */
> | 	ALIGN_SIZE_LOG2 = 3,
> | #else
> | 	/* All allocation sizes and addresses are aligned to 4 bytes. */
> | 	ALIGN_SIZE_LOG2 = 2,
> | #endif
> 
> 'ldrd/strd' require 8 byte alignment which might break with such
> alignment.

If you had asked me which alignment we have then I would have said it's
bigger. OTOH I never received any reports about insufficient alignment
on ARM or any other 32bit architecture.

I suspect we could just drop the check without any harm, but that's just
a gut feeling because we never had any alignment issues.

BTW are you sure ldrd/strd need 8 byte alignment? I just tested it with
the following patch and this works without problems. I verified the
compiler indeed generates ldrd/strd for accessing the 64bit field.

Sascha

-------------------------8<----------------------------

diff --git a/common/startup.c b/common/startup.c
index f53b73f81a..f261b1bdac 100644
--- a/common/startup.c
+++ b/common/startup.c
@@ -334,10 +334,31 @@ static void do_ctors(void)
 
 int (*barebox_main)(void);
 
+struct bar {
+	uint64_t foo;
+};
+
+struct bar *myfoo(void)
+{
+	struct bar *x;
+	void *ptr;
+
+	ptr = malloc(16);
+
+	ptr = (void *)((unsigned long)ptr | 4);
+
+	x = ptr;
+
+	x->foo = get_time_ns();
+
+	return x;
+}
+
 void __noreturn start_barebox(void)
 {
 	initcall_t *initcall;
 	int result;
+	struct bar *b;
 
 	if (!IS_ENABLED(CONFIG_SHELL_NONE) && IS_ENABLED(CONFIG_COMMAND_SUPPORT))
 		barebox_main = run_init;
@@ -355,6 +376,9 @@ void __noreturn start_barebox(void)
 
 	pr_debug("initcalls done\n");
 
+	b = myfoo();
+	printf("V: %lld\n", b->foo);
+
 	if (IS_ENABLED(CONFIG_SELFTEST_AUTORUN))
 		selftests_run();
 
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