Message ID | 20191014160404.19553-28-peter.maydell@linaro.org |
---|---|
State | Accepted |
Commit | f8ad2306d157a1db494dc0747f9f9afbae72cbda |
Headers | show
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[81.2.115.148]) by smtp.gmail.com with ESMTPSA id 5sm18029779wrk.86.2019.10.14.09.04.55 for <qemu-devel@nongnu.org> (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Oct 2019 09:04:56 -0700 (PDT) From: Peter Maydell <peter.maydell@linaro.org> To: qemu-devel@nongnu.org Subject: [PULL 27/68] target/arm/arm-semi: Correct comment about gdb syscall races Date: Mon, 14 Oct 2019 17:03:23 +0100 Message-Id: <20191014160404.19553-28-peter.maydell@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191014160404.19553-1-peter.maydell@linaro.org> References: <20191014160404.19553-1-peter.maydell@linaro.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. 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diff --git a/target/arm/arm-semi.c b/target/arm/arm-semi.c index 51b55816faf..302529f2278 100644 --- a/target/arm/arm-semi.c +++ b/target/arm/arm-semi.c @@ -217,10 +217,21 @@ static target_ulong arm_gdb_syscall(ARMCPU *cpu, gdb_syscall_complete_cb cb, gdb_do_syscallv(cb, fmt, va); va_end(va); - /* FIXME: we are implicitly relying on the syscall completing - * before this point, which is not guaranteed. We should - * put in an explicit synchronization between this and - * the callback function. + /* + * FIXME: in softmmu mode, the gdbstub will schedule our callback + * to occur, but will not actually call it to complete the syscall + * until after this function has returned and we are back in the + * CPU main loop. Therefore callers to this function must not + * do anything with its return value, because it is not necessarily + * the result of the syscall, but could just be the old value of X0. + * The only thing safe to do with this is that the callers of + * do_arm_semihosting() will write it straight back into X0. + * (In linux-user mode, the callback will have happened before + * gdb_do_syscallv() returns.) + * + * We should tidy this up so neither this function nor + * do_arm_semihosting() return a value, so the mistake of + * doing something with the return value is not possible to make. */ return is_a64(env) ? env->xregs[0] : env->regs[0];