Re: [PATCH v5 1/2] kexec: Support purgatories with .text.hot sections

From: Philipp Rudo <prudo@redhat.com>
To: Ricardo Ribalda <ribalda@chromium.org>
Cc: Eric Biederman <ebiederm@xmission.com>,
	Baoquan He <bhe@redhat.com>,
	kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
	Ross Zwisler <zwisler@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Simon Horman <horms@kernel.org>,
	stable@vger.kernel.org
Subject: Re: [PATCH v5 1/2] kexec: Support purgatories with .text.hot sections
Date: Mon, 3 Apr 2023 16:42:13 +0200	[thread overview]
Message-ID: <20230403164213.108093ec@rotkaeppchen> (raw)
In-Reply-To: <20230321-kexec_clang16-v5-1-5563bf7c4173@chromium.org>

Hi Ricardo,

On Thu, 30 Mar 2023 11:44:47 +0200
Ricardo Ribalda <ribalda@chromium.org> wrote:

> Clang16 links the purgatory text in two sections:
> 
>   [ 1] .text             PROGBITS         0000000000000000  00000040
>        00000000000011a1  0000000000000000  AX       0     0     16
>   [ 2] .rela.text        RELA             0000000000000000  00003498
>        0000000000000648  0000000000000018   I      24     1     8
>   ...
>   [17] .text.hot.        PROGBITS         0000000000000000  00003220
>        000000000000020b  0000000000000000  AX       0     0     1
>   [18] .rela.text.hot.   RELA             0000000000000000  00004428
>        0000000000000078  0000000000000018   I      24    17     8
> 
> And both of them have their range [sh_addr ... sh_addr+sh_size] on the
> area pointed by `e_entry`.
> 
> This causes that image->start is calculated twice, once for .text and
> another time for .text.hot. The second calculation leaves image->start
> in a random location.
> 
> Because of this, the system crashes immediately after:
> 
> kexec_core: Starting new kernel
> 
> Cc: stable@vger.kernel.org
> Fixes: 930457057abe ("kernel/kexec_file.c: split up __kexec_load_puragory")
> Reviewed-by: Ross Zwisler <zwisler@google.com>
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  kernel/kexec_file.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index f1a0e4e3fb5c..c7a0e51a6d87 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -901,10 +901,22 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
>  		}
>  
>  		offset = ALIGN(offset, align);
> +
> +		/*
> +		 * Check if the segment contains the entry point, if so,
> +		 * calculate the value of image->start based on it.
> +		 * If the compiler has produced more than one .text section
> +		 * (Eg: .text.hot), they are generally after the main .text
> +		 * section, and they shall not be used to calculate
> +		 * image->start. So do not re-calculate image->start if it
> +		 * is not set to the initial value, and warn the user so they
> +		 * have a chance to fix their purgatory's linker script.
> +		 */
>  		if (sechdrs[i].sh_flags & SHF_EXECINSTR &&
>  		    pi->ehdr->e_entry >= sechdrs[i].sh_addr &&
>  		    pi->ehdr->e_entry < (sechdrs[i].sh_addr
> -					 + sechdrs[i].sh_size)) {
> +					 + sechdrs[i].sh_size) &&
> +		    !WARN_ON(kbuf->image->start != pi->ehdr->e_entry)) {

Looks good to me. I'm not sure if it is better to use WARN_ON_ONCE to
avoid spamming the log when there are more than two .text sections or
when you reload the kernel. But that's only a rare corner case. So no
strong opinion from my side. Either way 

Reviewed-by: Philipp Rudo <prudo@redhat.com>

>  			kbuf->image->start -= sechdrs[i].sh_addr;
>  			kbuf->image->start += kbuf->mem + offset;
>  		}
> 