* Namhyung Kim <namhyung@xxxxxxxxx> wrote:
> The %pT format specifier is for stack backtrace. Its handler
> sprint_trace() does symbol lookup using (address-1) to ensure
> the address will not point outside of the function.
>
> If there is a tail-call to the function marked "noreturn",
> gcc optimized out the code after the call then causes saved
> return address points outside of the function (i.e. the start
> of the next function), so pollutes call trace somewhat.
> This patch will fix it.
>
> before:
> [ 18.345923] Call Trace:
> [ 18.346001] [<ffffffff812a8502>] panic+0x8c/0x18d
> [ 18.346257] [<ffffffffa000012a>] deep01+0x0/0x38 [test_panic] <--- bad
> [ 18.346347] [<ffffffff81104666>] proc_file_write+0x73/0x8d
> [ 18.346432] [<ffffffff811000b3>] proc_reg_write+0x8d/0xac
> [ 18.346516] [<ffffffff810c7d32>] vfs_write+0xa1/0xc5
> [ 18.346603] [<ffffffff810c7e0f>] sys_write+0x45/0x6c
> [ 18.346801] [<ffffffff8f02943b>] system_call_fastpath+0x16/0x1b
>
> after:
> [ 22.224483] Call Trace:
> [ 22.224569] [<ffffffff812bce69>] panic+0x8c/0x18d
> [ 22.224848] [<ffffffffa000012a>] panic_write+0x20/0x20 [test_panic] <--- ok
> [ 22.224979] [<ffffffff81115fab>] proc_file_write+0x73/0x8d
> [ 22.225089] [<ffffffff81111a5f>] proc_reg_write+0x8d/0xac
> [ 22.225199] [<ffffffff810d90ee>] vfs_write+0xa1/0xc5
> [ 22.225304] [<ffffffff810d91cb>] sys_write+0x45/0x6c
> [ 22.225408] [<ffffffff812c07fb>] system_call_fastpath+0x16/0x1b
Ok, this looks really useful - we really want 100% perfect backtraces, kernel
developers are looking at hundreds of thousands of call traces per year, so
every little detail helps in the long run!
[ Nit: please omit the timestamp prefixes from the changelog, they add no
information and just clutter the git log. ]
The implementation could be done a bit cleaner:
> +/**
> + * sprint_trace - Look up a kernel trace symbol and return it in a text buffer
> + * @buffer: buffer to be stored
> + * @address: address to lookup
> + *
> + * This function is for stack trace and does the same thing as sprint_symbol()
> + * but with modified/decreased @address. If there is a tail-call to the
> + * function marked "noreturn", gcc optimized out code after the call so that
> + * the stack-saved return address could point outside of the caller. This
> + * function ensures that kallsyms will find the original caller by decreasing
> + * @address and then adjusts the result by increasing offset.
> + *
> + * This function returns the number of bytes stored in @buffer.
> + */
> +int sprint_trace(char *buffer, unsigned long address)
> +{
> + char *modname;
> + const char *name;
> + unsigned long offset, size;
> + int len;
> +
> + name = kallsyms_lookup(address-1, &size, &offset, &modname, buffer);
> + if (!name)
> + return sprintf(buffer, "0x%lx", address);
> +
> + if (name != buffer)
> + strcpy(buffer, name);
> + len = strlen(buffer);
> + buffer += len;
> + offset++;
> +
> + if (modname)
> + len += sprintf(buffer, "+%#lx/%#lx [%s]",
> + offset, size, modname);
[ Nit: please do not break the line there, it makes the code less readable.
(ignore checkpatch in this case) ]
> + else
> + len += sprintf(buffer, "+%#lx/%#lx", offset, size);
> +
> + return len;
> +}
This is really just a trivial variant of sprint_symbol() AFAICS, to make
function return lookups more reliable. You look up address-1 then fix up the
resulting offset.
I'd suggest to introduce __sprint_symbol() internal helper function, with a
'symbol_offset' parameter to it.
That way sprint_symbol() can be implemented as a __sprint_symbol(..., 0) call,
while sprint_trace() can be implemented as a __sprint_symbol(.., -1) call.
Also, while at it, please rename sprint_trace() to something better. This is
not about tracing per se, this is about *backtraces* - and in particular this
is about return addresses to noreturn functions pointing outside the kallsyms
symbol of that function.
So a better name would be sprint_backtrace() or so?
> @@ -949,6 +951,7 @@ int kptr_restrict = 1;
> * - 'f' For simple symbolic function names without offset
> * - 'S' For symbolic direct pointers with offset
> * - 's' For symbolic direct pointers without offset
> + * - 'T' For backtraced symbolic direct pointers with offset
[ Nit: 'B' might be a better abbreviation for 'backtrace'. ]
Ingo
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