Re: [PATCH v4 01/25] rseq: Introduce feature size and alignment ELF auxiliary vector entries

On 2022-10-10 08:42, Florian Weimer wrote:

* Mathieu Desnoyers:

quoted

Export the rseq feature size supported by the kernel as well as the
required allocation alignment for the rseq per-thread area to user-space
through ELF auxiliary vector entries.

This is part of the extensible rseq ABI.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
  fs/binfmt_elf.c             | 5 +++++
  include/uapi/linux/auxvec.h | 2 ++
  include/uapi/linux/rseq.h   | 5 +++++
  3 files changed, 12 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 63c7ebb0da89..04fca1e4cbd2 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c

@@ -46,6 +46,7 @@
  #include <linux/cred.h>
  #include <linux/dax.h>
  #include <linux/uaccess.h>
+#include <linux/rseq.h>
  #include <asm/param.h>
  #include <asm/page.h>
  

@@ -288,6 +289,10 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
  	if (bprm->have_execfd) {
  		NEW_AUX_ENT(AT_EXECFD, bprm->execfd);
  	}
+#ifdef CONFIG_RSEQ
+	NEW_AUX_ENT(AT_RSEQ_FEATURE_SIZE, offsetof(struct rseq, end));
+	NEW_AUX_ENT(AT_RSEQ_ALIGN, __alignof__(struct rseq));
+#endif
  #undef NEW_AUX_ENT
  	/* AT_NULL is zero; clear the rest too */
  	memset(elf_info, 0, (char *)mm->saved_auxv +

diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index c7e502bf5a6f..6991c4b8ab18 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h

@@ -30,6 +30,8 @@
  				 * differ from AT_PLATFORM. */
  #define AT_RANDOM 25	/* address of 16 random bytes */
  #define AT_HWCAP2 26	/* extension of AT_HWCAP */
+#define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
+#define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
  
  #define AT_EXECFN  31	/* filename of program */

Do we need the alignment?  Or can we keep it perpetually at 32?  Or we
could steal some bits from AT_RSEQ_FEATURE_SIZE?  (Not the lower
bits—they aren't unused due to the way the feature size works.)

I cannot imagine a use-case that would require us to bump the alignment requirement over 32 bytes, so we may very well leave it at 32. But perhaps someone else has a better imagination than mine ?

Thanks,

Mathieu

Thanks,
Florian

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com