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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200526183853.GA24512@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Adhemerval Zanella via Libc-alpha Reply-To: Adhemerval Zanella Cc: libc-alpha@sourceware.org Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" On 26/05/2020 15:38, Szabolcs Nagy wrote: > The 05/26/2020 08:33, Adhemerval Zanella via Libc-alpha wrote: >> >> >> On 15/05/2020 11:40, Szabolcs Nagy wrote: >>> gcc -pg with -mbranch-protection=pac-ret passes signed return address >>> to _mcount, so _mcount now has to always strip pac from the frompc >>> since that's from user code that may be built with pac-ret. >>> >>> This is a backward incompatible _mcount abi change introduced by >>> return address signing support in gcc-7. >> >> Which change are you referring about specifically? > > gcc-7 introduced -msigned-return-address (which was > later deprecated by -mbranch-protection=pac-ret) and > the code generation with -pg is not compatible with > existing _mcount runtime implementations (because > now _mcount needs unconditional xpac) Thanks, could you add this explanation on commit message as well? > > i'm still trying to see if this _mcount change is > needed or we can change gcc to do the right thing > (and not pass signed address to _mcount), but i > need internal consensus about such abi break first. > >> >> Besides it, LGTM. >> >> Reviewed-by: Adhemerval Zanella >> >>> >>> TODO: fix -pg on the gcc side? >>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94791 >>> --- >>> sysdeps/aarch64/machine-gmon.h | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/sysdeps/aarch64/machine-gmon.h b/sysdeps/aarch64/machine-gmon.h >>> index 730a23b781..328cbdda16 100644 >>> --- a/sysdeps/aarch64/machine-gmon.h >>> +++ b/sysdeps/aarch64/machine-gmon.h >>> @@ -30,5 +30,5 @@ static inline void mcount_internal (u_long frompc, u_long selfpc) >>> #define MCOUNT \ >>> void __mcount (void *frompc) \ >>> { \ >>> - mcount_internal ((u_long) frompc, (u_long) RETURN_ADDRESS (0)); \ >>> + mcount_internal ((u_long) strip_pac (frompc), (u_long) RETURN_ADDRESS (0)); \ >>> } >>> >