linux-kernel - Re: [PATCH v2 1/1] perf tools: Check access permission when reading /proc/kcore file.

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Date:	Wed, 17 Jun 2015 22:56:08 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Li Zhang <zhlcindy@...ux.vnet.ibm.com>
Cc:	acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] perf tools: Check access permission when reading
 /proc/kcore file.

Li Zhang [zhlcindy@...ux.vnet.ibm.com] wrote:

| >For consistency with rest of the file, use pr_warning() or pr_err().
| 
| ui_warning can report the message to users directly when this
| program is running.
| But if we considered the consistency, pr_warning or pr_err should be better.
| And users can get this message by trying another time.

That seems to be the way perf currently operates - silent by default for
non-fatal errors. -v or -vvv increases verbosity and reports non-fatal
warnings/errors also.

| 
| >
| >Also, we could drop the access() call and report the error when open()
| >fails below?
| 
| I think we can drop this access. But /proc/kcore also require the
| process with CAP_SYS_RAWIO
| capability. Even if chown this file, access report right result, but
| open still fails.

Maybe the error message could hint that CAP_SYS_RAWIO would be needed.
| 
| >
| >|  	fd = open(kcore_filename, O_RDONLY);
| >|  	if (fd < 0)
| >|  		return -EINVAL;
| >
| >Further, if user specifies the file with --kallsyms and we are not
| >able to read it, we should treat it as a fatal error and exit - this
| >would be easer when parsing command line args.
| I have another patch which checks this files. I will merge it to this patch.
| 
| >
| >If user did not specify the option and we are proactively trying to
| >use /proc/kcore, we should not treat errors as fatal? i.e report
| >a warning message and continue without symbols?
| 
| In the current program, even if open fails, the program still
| continue to run.
| Is it helpful for users to get the address without symbols?

Well, if profiling applications, user may not care about kernel symbols,
so being unable to open /proc/kcore would be ok? If OTOH, user specifies
--kallsyms, then they care about the kenrel symbols so we should treat
the open() error () as fatal.

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